They are called asterina starfish. I used to have the same problem. They just took over. They reproduce by dropping of a leg that eventually grows into a full starfish. Mine started eating my coraline algae and were always on my zoanthids, keeping them closed. They were fine in small numbers. Once they got over the 100s I had to do something.
I got 2 Harlequin Shrimp that cleaned them up. I still have a couple left but they are only in my overflows, sump and fug. If you do go with the harlequin shrimp they only eat starfish. So once the asterinas are gone you will need to feed them a choc chip starfish once a month. Just my .02