This article is entitled Global Coral Bleaching Among the Worst Ever Seen"
Coral bleaching is when the coral loses its color. That color is due to algae which live on the coral. They provide energy to the coral through photosynthesis. They are said to be in symbiosis with the coral.
Coral bleaching is linked to global warming, specifically to ocean temperature. Increased ocean acidification due to higher concentrations of carbon dioxide in the oceans may also be contributing to the problem.