Look like flatworms to me (the picture's kind of fuzzy). Haven't had them in my tank because I try and avoid purchasing from places that have obvious flatworm problems, either that, or somebody I have is eating them. I know there are a couple of species out there that do eat them, but I'm not sure what they are. Maybe a slug? Can't remember. If I think of it, I'll tell you. Otherwise, do a search for flatworm on the internet or on this site.
By the way, if they are flatworms, they will spread. They'll cover your substrate, your glass, your corals, your snails, you name it, they'll be there.
[ August 17, 2001: Message edited by: davelin315 ]