It looks like aiptasia. The best way to remove big ones is with aiptasia x. Or you could get a aiptasia eating file fish. My lfs let me rent one so it could eat all the aiptasia in my tank then when it had no more to eat I took it back to the fish shop. You could keep it, they do look quite nice. But then it may override your tank, if you where wanting to kill it the methods I mentioned will work or I'm sure some other people have ideas. Do not use fire though. Hope this helps.