+1 these are most likely Asterina stars, I have them anywhere from 1mm - 12mm in size and sometimes there are hundreds and sometimes I can't find any. I have tried to pick them out, but I don't think that you can every get them all manually. One the plus side, I have never seen them bother any of my Zoas or any other coral only feeding on the alga on the sides of the glass and in the sand. I have heard that if you know someone who has a Harlequin shrimp, try to borrow it for awhile. They only eat starfish and will remove these pretty quickly.