A sign of coral reef distress is called

Coral Bleaching

Explanation:

"When water is too warm, corals will expel the algae living in their tissues causing the coral to turn completely white. This is called coral bleaching. When a coral bleaches, it is not dead. Corals can survive a bleaching event, but they are under more stress and are subject to mortality."

- NOAA's National Ocean Service

Hope it helps!

A is obviously the answer