If it's an elysia it's probably more of an algae sucker than an algae grazer. I have an elysia crispata and I read everything I could find on it on the sea slug forum. Apparently they are sacoglossans, which means that they only eat certain kinds of algae that have large cells, such as bryopsis and a few others. They pop the cells and suck them dry or something. At least that's what I remember of what a read a few weeks ago. I hope I got it right.
Anyway, it's not going to mow down hair algae like a sea hare. Holy crap, has anyone seen what those things did to that tank at Skipton's that used to be clogged with hair algae? I couldn't believe it when I went in to get water today.
Back to the elysia -- mine seems to change size over quite a large range. I was worried about him shortly after getting him, since he seemed so small and withered, which is why I scoured the sea slug forum. But sometimes he just sits there all shriveled at less than an inch and other times he's out cruising the tank at a pretty good clip, looking better than two inches. Neat creature.
Anyway, enjoy him. Don't let him eat your dog. And I wouldn't let your dog eat him either -- he might be poisonous. I think mine did in the first barnacle blenny I had. Lenny the blenny was the happiest, healthiest little guy. And a riot to watch at feeding time -- unbelievable speed, even when backing into his hole in the rock. I noticed one morning after feeding that the crispata happened to park for the day right above Lenny's hole. Lenny kept eyeing him nervously, as he did everything around his hole. Anyway, when I got home that evening Lenny wasn't doing well at all. He looked like he was drugged or something and was lolling around on the bottom, starting to get picked on by the hermits and snails. I suspect he made the mistake of chomping on the elysia.
--cn