second GSMguy
Aiptasia Anemone and some people call it Rock or Glass anemone. They can be a weed if you don't take care of that one. Don't try to kill it by using any tool, like beating it, scrubing it because you will end up with many more. You need either to take that rock out (if is small) and leave it out for a while untill it dries and put it back. If the rock is big then you can use Kalkwasser if you have any (thick paste) and put it on it. Make sure Kalk does not get on any corals if you have any. If you have alot of them then don't kill of of the in one day using Kalk without checking your PH. You can also use natural predators like Berghia nudibranches can be used but if they don't find any more then starve. Peppermint shrimp, Chelmon rostratus, the Copperbanded butterfly fish also keeps them under control if not finishing them, Empror angel fish, Joe's Joice, Red Legged Hermit Crab,