I agree somewhat with Nifft and others on some of the traits, but pointedly disagree on some other things....... The prehensile tail should not help with swimming; it's not like an eel's tail, nor a fish's, nor a lizard's; it lacks the width and strength to help a humanoid swim better, far as I can imagine it. Nor should the tail grant a Dexterity bonus; it doesn't make them all-around better coordinated, poised, flexible, reflexive, or the like. I'd say the tail would grant +2 on Balance, Escape Artist, Sleight of Hand, and Tumble checks when the tail is useable, +4 on Climb checks when the tail is useable, and +2 on grapple checks when the tail is useable; I don't see any way that a tail like that could help significantly against bull rushes or trips, and it probably isn't as good as a lizardfolk's tail for balance, nor is it significantly strong enough or huge enough to make the character effectively one size larger for grappling (which is what a +4 on grapple checks would be like).
I don't think they should have a climb speed, but I'm not sure yet if it's truly inappropriate for them. Having a tail should not prevent the character from wearing armor, nor should it make armor 50% more expensive just to put a hole over the tail area. Even full plate shouldn't be a problem. I'd say add +10% to the cost of clothes and armor for the tailed race, in order to have a hole made around the tail area; the tail won't be armored, but no matter, it's probably agile enough and quick enough to avoid getting hit, and you can't target specific body parts normally in D&D anyway so it doesn't matter.
A prehensile tail would not incur an ECL+ in and of itself, it depends on what other abilities they have, and how they can use their tail. Chiefly, the tail could act as a third arm and hand, using weapons, shields, wands, scrolls, tools, and so on. If strong enough to hold the character's weight, it should probably grant them a tail slap natural attack appropriate to their size; probably 1d4 damage if Medium, 1d3 if Small, applying one-and-a-half times their Strength bonus to damage, if any, since it's their only natural attack (as per the MM). Being able to do anything a hand could is pretty significant though, so you may consider disallowing its use of weapons and shields, saying that it can't grip and hold up such devices well enough to strike or block with. A tail doesn't have fingers after all, so there are a few things it couldn't do like a hand. Couldn't play most instruments (besides drums), but could hold them at least, for instance.
Now then, evaluating the racial stats you've posted...... In my opinion, what you have posted at the moment is appropriate enough for a race with no Level Adjustment, in terms of usefulness, but it is on the stronger side of LA +0. As-is they make much better rogues than Halflings and Elves do, and I don't really agree with some of their stats, though they are more or less balanced as-is. Also, you misspelled Treant and Rogue near the end. I'd modify 'em a bit like this:
Celiran
* +2 Dexterity, -2 Strength
* Medium-sized
* Humanoid (Celiran)
* Celiran base speed is 30 feet
* Prehensile Tail: A celiran's tail can be used for almost anything that an arm and hand could be used for, with the following exceptions. The tail's lack of fingers, combined with its slightly lesser coordination than a true hand, and its lack of a true arm's bony support renders the tail less than adequate for certain tasks. It cannot use a weapon or shield, though it can hold one such item idly and uselessly. The tail cannot perform the somatic components for spells or other abilities. It cannot play any instruments that require the use of more than one finger (the tail can effectively hold something and simultaneously function as one finger, for purposes of manipulation), nor can it use tools that require multiple fingers. A celiran's tail does not count as a magic item slot for shoulder, arm, hand, or finger items, with the exception that the Extra Item Slot epic feat can be applied to it as such.
* Tail Slap: Celirans may use their tail as a natural weapon, dealing 1d4 damage with a tail slap natural attack. As their only natural weapon, it applies 1-1/2 times their Strength bonus to damage, if any. As with other natural weapons, it threatens a critical hit on a roll of natural 20, and deals double damage on a critical hit. Celirans are, naturally, proficient in their tail slap. This tail slap may be used alone, or may be used instead as a secondary attack during a full-attack action. In the latter case, it suffers a -5 penalty on the attack roll and applies only 1/2 the celiran's Strength bonus to damage, if any.
* +2 racial bonus on Balance, Escape Artist, and Tumble skill checks, as well as on grapple checks, when using their tail to help with the task
* +2 racial bonus Sleight of Hand checks when able to use their tail, whether it is to help with the task or when using the tail exclusively
* +2 racial bonus on Climb checks, increasing to +4 when using their tail to help
* Tree-Dwelling: Celirans gain Climb as a bonus class skill with all classes they take, and they can always take 10 on Climb checks, even under duress
* Natural Camouflage: Celirans gain a +2 racial bonus on Hide checks in green or blue surroundings, due to the coloration of their scales
* Tail Accomodation: A celiran needs to wear armor or clothes that accomodate their tail, as it can't fit in typical clothes made for other humanoids their size. Celirans must spend 10% more on armor and clothes when buying or trading for such goods not tailor-made to fit celirans in the first place. This assumes that the armor or clothing is made to allow the tail's freedom of movement while still covering it partially or fully, but a hole can be cut in clothes and cloth-like armor (padded, leather, studded leather, hide, or the like) at no extra cost instead. Harder armors, such as those incorporating more metal than studded leather does, cannot have such a hole cut, and must instead be modified by an armor smith with a forge and tools to accomodate a hole, labor that costs 5% of the armor's value and takes 1 hour. It takes a day of such work, costing 10% of the armor's value, to make armor that protects the tail and suitably fits a celiran, without impeding use of the tail any moreso than the armor check penalty already does. This 10% cost increase applies even when the armor is made for celirans in the first place, if made to protect the tail too. However, there is no game penalty for leaving the tail unprotected.
* Automatic Languages: Common, Ceran.
* Bonus Languages: Draconic, Elven (all three variants count as separate languages), Goblin, Old Dwarven, Orcish, Treant.
* Favored Class: Rogue.
