I didn't know what these were called until I read this thread. I have a ton of these in my tank also. I Googled the name and found many websites that say these could bother/eat/hurt many corals, the major names were polyps, zoos and some leather corals. I think I will do a little more research and maybe get one of those starfish eating shrimp. This might be the reason I have never been able to grow zoos or polyps