Simple answer:
When corals are stressed by changes in conditions such as temperature, light, or nutrients, they expel the symbiotic algae living in their tissues, causing them to turn completely white. Warmer water temperatures can result in coral bleaching. ... This is called coral bleaching. When a coral bleaches, it is not dead.
More in depth explanation:
Exposure to higher (100 and 1000 µg l-1) diuron concentrations for 96 h caused a reduction in ΔF/Fm¹, the ratio variable to maximal fluorescence (Fv/Fm), a significant loss of symbiotic dinoflagellates and pronounced tissue retraction, causing the corals to pale or bleach.
Hope this helped :)
