The following is from The Reef Aquarium vol. 1.
Loss of the symbiotic zooxanthellae make the coral...look white, pale yellow.. as if it had been dipped in bleach. The tissue is still alive but now it is transparent. In some cases there is no actual reduction in zooxanthellae number but a decrease in the pigment content of the zooxanthellae. This is related to changes in light intensity... Bleaching can occure suddenlyif the coral has been exposed too long to water that is too hot or too cold, or if the light field has changed too radically.
Hope this helps.