Probably a nudibranch of some sort - especially if it reminds you of a slug. If you have corals, it could be a problem as some like to munch on coral polyps. From what I understand though, nudibranches are fairly specialized feeders and if you don't stumble across exactly what it is that it likes to eat, they'll die of starvation. So I'm guessing your hitchhiker might not be around too long. Perhaps a snail/slug expert can chime in!