Sounds like it has bleached...
Found this handy explanation:
"Bleaching: disease of some animals with symbiotic zooxanthellae. The disease is triggered by a stress, often a thermic stress (environmental temperature variation). The animal reacts getting free of the zooxanthellae. We can observe tha animal becoming white because usually the tissue of those species are completely transparent, as a result of the co-evolution with a symbiotic alga, needing full light to live. The colour of the animals with symbiotic zooxanthellae is actually the colour of the algae.
The bleaching means not automatically the death of the animal, normally a recover is possible (if the stress factor is removed)."