Why is some of the coral in the great barrier reef white?

Algae is the main reason for the color of the corals and when algae is not present, the corals become white.

Explanation:

The rising temperatures of sea water are the reason why some corals of the Great Barrier Reef are white. Higher temperatures of water cause bleaching of the corals. During bleaching, the corals lose their color because the coral polyps expel the algae that live in them.  Algae not only provide color but also energy to the corals and after bleaching they may not be active.