Deano, I think what the guys are saying is that it appears to be an asterina starfish being attacked by a seaslug.
Jeff, is that pic of 2 animals or 1? If it is an asterina, it is the weirdest asterina I have ever seen. But if it is an asterina with another animal on it, then yeah it's an asterina.
I would take it out, unless you want your tank over runned by those little buggers. They multiply like crazy. The only way I was able to controll them was with a Harlequin Shrimp.