Frankie - see thats whats puzzling me - he has this "generic" look about him that I cant decide if hes just a blasted nudi or one of the elysia species. But if the elysia species eats algae I havent seen him eat any, unless the kind of algae he eats ISNT the brown/hair algae thats been growing in our tank. And he seems to like to get up on a perch and raise up like that as if he's sniffing out something. I can only figure he's smelling the coral (if thats possible) and trying to figure out where to get to it.
Yes, that is what it is doing - sea hares, sea slugs, flat worms and nudibranchs - all the species that have rhinophores are in essence sniffing chemicals in the water column. On the species in photo provided the rhinophores are the tall black tentacles at the front part of the body, these organs have special receptors to help them find food.
As for ID of this critter, I'd group it in with the sea slugs, not in the Nudibranchia Order b/c of its morphology. It doesn't have an aeolid nudi anatomy, which has a mantle that is extended into long finger-like projections called cerata. And for the dorid nudibranchs anatomy, the mantle is thick and extends over the foot, which it looks like this critter has, but the surface of the mantle may bear tubercles which vary in size, shape and number and are often a character used to identify nudibranchs and this critter doesn't have these and it doesn't have extended gills on the rump.
I also looked into sea hares, but again the anatomy doesn't match-up b/c they usually have a head bearing a pair of enrolled rhinophores, and large flattened, enrolled oral tentacles on each side of the mouth. This critter is missing the oral tentacles.
So, that leaves us with sea slug or flat worm. I'm leaning towards sea slug. All these species are very selective on what they eat, you may not have the prey items it is looking for in the tank, which means that it is possible that it will slowly starve to death. As an experiment you can add several different types of algae to a clip and see if it goes for it....
There are thousands of nudis, sea hares, slugs, etc. and little is known about many of them and less about their diets. From what we do know, they tend to have very specific diets, a particular species can spend its whole life on one coral head and eat from that one species of coral only. The ones that show up in our aquariums and cause havoc are the ones we tend to hear more about, many more end up dying when our live rocks cycle and we don't even know it.
Another thing to keep in mind is that their life cycle is very short and most won't live past a year, some live only three months... what becomes an issue is that they are hermaphroditic, so they can easily reproduce in our tanks. Again, we hear more about the little ones which eat our corals and tend to reproduce and cause havoc in our tanks.
