a whiff of grapeshot

I've had this coral for about 3 months and it was doing great. I fragged a piece off and it was doing good in my tank too. In the lastfew days I've noticed that it started to slump over and it's stopped opening up. This morning I found it like this. I tested my water and everything is fine; pH is within range, ammonia is 0, nitrite is 0, nitrate close to none, and phosphate is 0. The salinity is also within range though I've never really had problems with that. There is some discoloration happening ...