There are thousands of forms and colours of coral in the reef. Unluckily, when the water gets too warm, they shed symbiotic algae (but don't die, at least up to a certain limit) and become white, which is referred to as "Coral Bleaching". The photo was taken during the "Coral Bleaching" event of 2002, most corals in the area were affected by it. We do not wish to enter the discussion whether this was man-made (well, probably it was), but there are other man-made dangers to the reef, like overfishing and pollution, but also natural ones, like the Crown-of-thorns starfish.