What it is, what it looks like and what causes it.
Corals (which are animals) have microscopic marine algae (plants called zooxanthellae) living inside their tissue. These give corals their colour and food.
When corals are under stress, they expel the zooxanthellae. Without these, the coral’s tissue becomes transparent and the bright white skeleton is revealed. This is coral bleaching.
When a coral bleaches, it’s not dead. But if the stress is prolonged, bleached corals begin to starve without their food, and will eventually die if the stress is not relieved.
Warmer water temperatures than normal cause corals stress.
Mass coral bleaching happens when the ocean stays too warm for too long. If water temperatures return to normal quickly enough, corals can recover and their resident zooxanthellae will move back in. If the water temperature stays hot for a long period, the bleached coral cannot survive.