Coral bleaching. If the stress-caused bleaching is not severe, coral have been known to recover. If the algae loss is prolonged and the stress continues, coral eventually dies
NOAA:  National Ocean Service
2020-01-07
<a rel="nofollow" class="external text" href="https://oceanservice.noaa.gov/facts/coral_bleach.html">NOAA:  National Ocean Service</a>