Well some folks will say no way they are harmless but I can tell you without a shadow of doubt that they are not. I have on 4 occasions now caught the small 3 legged gray / blue Astrena star fish killing my Zoa / Paly Polyps. Today I saw 1 polyp in the center of my 20 polyp PPE colony closed up. Upon further inspection there was a gray / blue astrena latched on to the polyp. On other occasions I have looked in my tank to see a clear spot on a Zoa colony and found one on what was left of the polyps. Now these are not the white ones that normally hang out on the glass. These are a slate gray / blue color and usually have 3 - 3.5 legs. They tend to stay on the LR and then show up on my Zoa / Paly colonies. You have to look pretty close as they tend to blend in on the LR. Be afraid...Be very afraid.