Not really enough detail to be sure..... I guess it's a ribbon worm (nemertean). The worm looks smooth without the annulations that distinguish polychaetes. the way that it moves is typical of the "flow" that some nemerteans show when they move.
Nemerteans are predators on small organisms like other worms, tiny crustaceans, etc. You don't have to worry about it attacking corals or clams.
Thanks as always Leslie - I thought it was a ribbon worm because of the undulating movement but wasn't sure. I've seen several of them in my sump/refugium and since I don't kill anything they can stay there and do their thing, I'm sure there is something i there that eats them as well.