Why do corals bleach white?

Answer: Stress expels symbiotic algae

Disease from bacteria — Corals do get bacterial diseases, which cause bands of dead tissue or lesions, but bleaching is a different event: the coral is alive and simply lost its algae in response to environmental stress. A bleached coral can still recover, which a coral killed by disease cannot.

Stress expels symbiotic algae ✓ — Correct! A coral's color and much of its food come from zooxanthellae, microscopic algae living inside its tissue. When the water gets too warm, too polluted or too acidic, the partnership breaks down and the coral expels the algae, leaving its white skeleton visible through the clear tissue. If the stress eases soon, algae can return and the coral recovers; if it drags on, the coral slowly starves and can die.

Absorbing too much sunlight — Strong sunlight does add to the stress, especially together with warm water, but bleaching is not the coral fading like a cloth left in the sun. The whiteness appears because the colored algae have been expelled; sunlight is one of the triggers, not the color change itself.