Smitty is right. Definitely majano anemone. They will wipe out your livestock, so get rid of them ASAP. You can use Aiptasia X to kill them. I had my old 240 over run with majano. I started with one, and they quickly multiplied in the thousands and killed every coral they touched. I ended up having to boil 500 lbs of rock to kill them. And I still get them popping up from time to time on the old rocks! They are incredibly hardy, and really horrible pests!