Coral turns ghost-white when it evicts the algae that feed it

Reef-building coral survives by hosting microscopic algae in its tissue, which photosynthesize and share food with the coral in exchange for shelter — and give coral its colour. When water gets too warm, the coral expels these algae in a stress response, leaving its white skeleton visible through transparent tissue. The coral isn't dead, but it's starving, and if warm conditions persist too long it will not recover.

— Ove Hoegh-Guldberg, Coral Bleaching: Causes and Consequences — Coral Reefs journal, 1999