"Green Star Polyp(Pachyclavularia violacea) is an encrusting type coral that has bright green polyps or occasionally light brown or gray. It has a large bright center to each polyp and when the are retracted there are dark purple, tubular calcyes left projecting above the mat. A similar species, Briareum sp. will leave only calcyes only slightly projecting."
Anthelia usually have longer stolon and grow much like xenia. Not all star polyps are smooth, Briareum are not as smooth as Pachyclavularia. However I see where you are thinking it could still be anthelia here. So there is an easier way to tell and resolve this quickly, take a picture of it before the polyps come out and post that here. If you like just wave your hands over it a few times until the polyps retract then take the picture.
Sorry to do this but: There is yet another coral that this resembles, encrusting gorgonia look very much like this as well.