Hard to tell. Given that it appears to be in a shaded area, my guess would be a ball tipped anemone, which are harmless. That said, aiptasia are not necessarily the curse they are made out to be. There are certain kinds that spread more aggressively than others. I had one aiptasia in my nano that I just watched it for 18 months and never multiplied. After the transfer to my ghetto tank. I noticed 3, the original one and 2 on recently acquired frag plugs, so likely they came into my system recently. Because I was not sure of these new ones, I got a peppermint and now I have no aiptasia as far as I know. Hope this helps.