Perfect blue, The last picture posted went way to large
and kicked everything over, No reply, edit or other
functions. If you could delete picture or post to fix
it. Thanks, Eznet2u

It looks like a mini stafish to me. And they are good, not bad :D

I resized the picture and although it is hard to tell from the picture it does in fact look like a tiny Starfish. From the picture I can't make an exact identification so I'm not sure if it is beneficial or harmful to your tank. I have although seen pictures of Starfish that resemble yours and they have been known to eat corals.