I'm not certain but I'd get it out. It looks a lot like some sort of nudibranch but I can't really tell. Usually small things that colored are flatworms of one type or another but this little critter looks like it's got a distinct and snail-like head. Is it smoother are are there "frills" on it's back? That would point towards nudibranch. Does it seem to have a head and move like a snail? There are a few good nudibranchs out there and a lot of bad one's. I'd err on the side of caution and segregate the thing at the very least.