Firstly, a regular Nudibranch collapses under its own weight out of the water. It should not be able to move on land and might require some trait to reflect that.
Also, it needs
Water Breathing to prevent it drowning!
Sea slugs are really squishy so I don't see why it would have much natural armor. Indeed, I'd suspect its AC would be very unimpressive, possibly even 10 or less. Its main defense it its stings and being a bucket of hit points (IIRC sea slugs can take a lot of damage without dying, they're just easy to damage).
They're also
extremely slow sine they move like, well, slugs! A pelagic nudibranch like a blue glaucus basically just drifts about on the current. It can swim under its own power, but it's not moving as fast as a shark! I'd think
Speed 10 ft., swim 20 ft. would be more accurate.
Ability wise, the DEX looks way too high as Nudibranches are hardly noted for their speed and agility. I'd think something in the range 3 to 7 would be better. I doubt it would any higher than, say, a Purple Worm's DEX 7 and it could easily be lower.
The
Keen Scent is fine, but I'd also add:
Water Breathing. The blue glaucus can breathe only underwater.
Waterbound. A sea slug can only use its ground speed when submerged in water.
Rather than giving it Multiattack with up to six "Appendages" I'd suggest making it a trait, so any creature venturing too close to the creature must avoid contacting one of its cerata and getting stung.
Indeed, I wouldn't give it Multiattack at all.
I have no objection to having it use its Cerata as a action too. So maybe something like this:
Traits
Venomous Defense #1. Any creature within 15 ft. of the blue glaucus must make a DC 13 Dexterity saving throw on each of its turn, if it fails the blue glaucus can make a stinging cerata attack against the creature as a free action.
Venomous Defense #2. Any creature within 15 ft. of the blue glaucus must make a DC 13 Dexterity saving throw on each of its turn, if it fails the blue glaucus automatically hits the creature with a stinging cerata attack.
Actions
Stinging Cerata. Melee Weapon Attack: +6 to hit, reach 15 ft., one target. Hit: the target must make a DC 13 Constitution saving throw, on a failed save they take # (#d# + #) poison damage and are poisoned for 1 hour, on a successful one they take half as much damage and are not poisoned.
Of those, I prefer
Defense #2.