Almost looks like a Bubble-tip anemone but I'd guess it's probably some kind of Rhodactis.
I have some of these in my tank and they're almost impossible to get rid of ,,, they grow/split like crazy under MH. They've stung some of my SPS' pretty badly on occasion.
They are currently hosting a pair of black and white Oscellaris who seem quite happy with them.
They get pretty big,, here are some of mine,,, the carpet anemone in the background is about 12" in diameter.