pseudocorynactis coboverdensis or pseudocorynactis caribbeorum.
A not fully studied coralimorpharian species. Some kind of rock anemone that I and apparently many others have randomly in their tanks. Harmless as far as I know, but watch their size, one kind can get rather large and then may be an issue (unknown). Thank reef encyclopedia Nick for the info (I stole it from him
) HTH