I had a gsp do the same thing too after about 6 months in my tank. Never did open back up and just slowly shrank away to nothing. everything else in the tank was just fine. Go figure? Also had pulsing zenia do the same thing. Heard both of these corals are like weeds and grow like crazy, you can never get rid of them. Brown thumb i guess lol

It might have been your mag. My Zenias were doing the same thing but then I changed salts with more mag (after being told to try this by one of my lfs) and now I am always trimming back the Zenias. (Of course assuming your ph is above 8.1)