Looks to be dying to me too... I've had a hammer do that in the past, but it was because my tangs eat the gravel, poop it out on the coral, and with not enough flow, the gravel accumulates, and keeps it from expanding, killing parts of it. I upped my flow, now no problems. This happened years ago before I figured out why it would be the only part of a coral going down hill. Not saying that's your problem, but that was mine a while back.