By the way, I am trying an experiment in caulerpa control in a 54. This pernicious weed was never my choice. It came with my rocks, it survived a move and accidental rock cooking that killed everything else but one aiptasia---and it is nasty!
So, after scouring Foster/Smith's very good critter descriptions, I lit on a sea hare as a possible caulerpa eater. Mr Kumagoro is about the size of a golf ball, and while they look quite disgusting in pix, they are cute to the max in motions and habit. I cannot swear whether he is doing anything about the plague, but he is pooping and he is buried in a bad patch caulerpa all day and night. I can't figure out where his mouth is---but in a tank with mostly gobies and no notable aggressors, he's been quite serenely at his job, and the fish do not seem remotely interested in bothering him. As you may know, they ink like an octopus if stressed, and allegedly puff up and fly about on the current like a 'nem when deciding to travel, so you do need to block any potential exits and shield any intakes, but so far he is a good citizen and far from causing any problem. Stand by for more on the sea rabbit, as soon as I find out if he's actually eating the stuff.