Just so happens that I have a pic of one of my asterina stars. This one is about the size of a nickle. My tank has a lot of these little guys, not all that big, and I haven't seen a problem at all with them. I think they're really cool. As a matter of fact, my daughter and I have been thinking of scooping some out of my tank to put in hers...
Notice the little one to the left of the filter siphon...five legs...some have three, some four...very irregular.
Lisa :happyfish