Seems pretty good to me right there. An idea: Why not make the slam secondary since it doesn't use it much?
I mean secondary as in the normal definition "Attacks with secondary natural weapons are less effective and are made with a –5 penalty on the attack roll, no matter how many there are. (Creatures with the Multiattack feat take only a –2 penalty on secondary attacks.) This penalty applies even when the creature makes a single attack with the secondary weapon as part of the attack action or as an attack of opportunity."
But I suppose you could maybe talk me into no penalty on a charge. I think it should have the penalty on a normal full attack, though.
Yeah, let's keep it secondary. It can always choose to attack with it as its sole attack if it prefers.
How about giving it improved grab with its whips, and something akin to gnaw where it gets a free charging slam attack (with powerful charge) if it hits with both whips?
We've already got far too many creatures with Improved Grab, so I'd rather not add another one. Slamming foes with charging head-butts is its schtick.
Furthermore, I think of Marrella as being more of a scavenger than a predator.
Besides, if its whips have a 10 foot reach how would it have enough distance to charge an opponent it's grabbed with them?
I'm ok with leaving them as is. As scavengers, they don't need to be the toughest combatants.