I can't say 100% cetain as it depends on where your live rock is from and a better pic. But this is probably Anemonia viridis commonly called Snakelocks anemone. It unfortunately is a pest.
They came from some figi rock. The little anemones are about 1/2 round and 3/4 inch tall. They keep moving all over the tank. When I got up this morniing I thought that two of them had died, but a couple hours later they both split into two more. 8O I think I am going to get them out of my tank..............
