This bleached coral appears as white due to damage done to the Zooxanthellae (zoo-zan-thel-ay) with which it has a mutual symbiotic relationship.
Zooxanthellae are actually responsible for giving corals their vibrant colors. They are unicellular algae that live within the hard coral.
Corals are unable to generate sufficient amounts of food, so they utilize the algae’s ability to photosynthesize and convert chemical elements into energy to supply them with the energy they need. In turn, the coral offers the algae a protected, nutrient-rich place to grow.
When coral is bleached, it means that the Zooxanthellae that lived within it has died; this, in turn, means that the coral will die.