Any Ideas??

Just spotted this little thing on my rock this afternoon. Does anyone know what it could be??

Have no idea unless its some kind of flatworm.

Is that a shell?

from what i can gather from the picture it looks to me that it is the top of a Asterina Anomal tiny starfish that are controversially looked down on for claims to eating corals. however other evidence suggests that when observed doing such activities they are feeding on already dying tissue of a coral. wither way thats what it looks like to me. some of mine have that really grey look to them.

http://www.chucksaddiction.com/hitchstars.html

Yeah, I noticed it moving very slowly across that same rock, so maybe it IS a tiny starfish.

+1 Alex. I too have asterina's and found the same controversial info you mentioned. I chose to keep them and now I have several of them and they have caused absolutely no harm. They have been in for +/- 6 months and I've never seen them near my corals much less on them. I usually only see them on the overflows or on the LR and I spend time every day looking over my tank.