I believe it is a majano anemone. They are a super pain in the ***. They will spread fast, get him out of there if it matches pictures on the net of majano anemones, I have had issues with these guys in the past.
If you start to see several pop up. A matted filefish will nail those puppies after adjusting to your tank. My experiences have been good with matted filefish not going after corals, however, I have an sps dominated tank, so ask around about them in an LPS tank