Who knows what it could be, but it seems it is either a poly or a rock anemone. It has lime green tiny tentacles around the edge and it is much clearer in colour in the middle with what looks like lines in the form of a wagon wheel. It also retracts to Hal the size when lights go blue or are off. Can't take a better pic yet because it's closed up now but will try another time when it's fully opened up.