Coral Bleaching is nothing more than fear-mongering hype aimed at scientifically illiterate gullibles.
The idea being sold in the Coral Bleaching hype is that Global Warming is increasing the ocean's temperature which is causing the ocean to become caustic and to become a hostile environment for the corals which succumb and die, turning white as a ghost.
This alarmist hype inappropriately uses the word "bleaching" in place of "whitening" to invoke imagery of caustic chemicals somehow being involved. In English, whitening can occur from bleaching but bleaching always involves bleach. In the case of corals, whitening happens and is not a result of any bleach so the name "Coral Bleaching" is itself a lie.
Coral polyps are mostly clear but otherwise would appear very white in large clusters. Nonetheless corals are famous for their bright colors. These colors come from the millions of zooxanthellae (tiny algae) living inside every square inch of their tissues, producing all the pigments. Whenever currents bring in a rapid change in temperature or the water temperature becomes warmer than normal, the natural reaction for the coral is to jettison all the zooxanthellae, thus becoming bone white.
The coral does not die. The zooxanthellae return and within a few weeks the coral is back to being colorful.