“Anthozoa” literally means “animal flowers” (from Greek ánthos “flower” and zóa “animals”) a reference to the floral aspect that octopuses assume. Coral, perceived as a single organism, is made up of thousands of identical individuals genetically called polyps.
When there is an increase in temperature, the whole structure enters a sort of “fever”, the polyps are no longer receiving food so they starve. 
The bleaching of corals is a tangible sign of the reaction of these to some form of stress, especially to the increase in water temperature.