Explanation:

The whitening oi coral colonies due to the loss of symbiotic zooxanthellae irom the tissues of polyps is called as Coral Bleaching.
Zooxanthellae are unicellular algae that provide colour to corals. They also provide food to corals and their deaths lead to breakdown of the symbiotic relationship between them. This breakdown of symbiotic relationship causes starvation and resultant deaths of coral polyps.
This condition exposes the white calcium carbonate skeletons of the coral colony. There are a number of stresses or environmental changes that may cause bleaching.
These causes include disease, excess shade, increased levels of ultraviolet radiation, sedimentation, pollution, salinity changes, and increased temperatures etc.