Reef-building corals host dinoflagellate algae of the family Symbiodiniaceae inside their cells. The algae photosynthesise and pass a large share of the resulting carbon to the coral, in most cases the majority of its energy budget. They also supply the coral's colour; coral tissue itself is largely transparent.
When water temperature rises a degree or so above the local summer maximum for a sustained period, the symbiosis breaks down and the coral expels the algae. The white appearance is bare aragonite skeleton visible through clear living tissue. The polyp is alive at that point and can still feed on plankton, but at a fraction of its usual energy intake.
Recovery is possible if conditions cool and symbionts are reacquired within weeks. Prolonged bleaching leads to starvation, disease and death, after which the skeleton is quickly overgrown by turf algae. Distinguishing the two states matters for reporting: a bleaching event is a measurement of stress, and the mortality figure that follows is a separate observation made later. Bleaching can also be triggered by cold shock, freshwater and light stress, not heat alone.