That looks like either a flatworm or a nudibranch. A picture of the top side would help.
I would take it out and put it in a container and post a dorsal view so people can identify it. Possibly a bad tank mate?
you guys are right after looking at all the nudi online i have found it .. its defiantly a Tritoniopsis elegans, i have lost it in the sand but i need to find it somehow and get ride of it .. i dont have corals at the moment ..
alot of people in R2R reporting that it eats soft coral ...