ahhh, i had those at one time and i was flippng out, calling everyone but couildnt get a pic like that.
the best explanation i got were baby starfish, and they were gone within a week, they'll get eaten if thats what they are
i was worried at first it was aiptasia,or baby apt, but they dissapeared and i have never seen them again
I had those popping up on the glass on my tank like nuts for about a month and a half after I first set it up. Occasionally I still get some. Most I just wiped off with my magna float thing but the ones on the back I didn't wipe off appeared to have turned into a sort of curled up feather duster. I don't think they are a bad thing, maybe just a juvinile form of that species.