The grey ones are a different species. Usually the ones that are dead are white. 

The temperature in the region had been 34 in the summer, so there has been quite a bit of coral bleaching.

https://cdn.satellite.earth/2d12e20b3c34fa2990dfeebf76b58f8ea0131a728bf7103a4da4277f36e48435.jpg

On the left side of this picture, the coral is dead i believe.