Well Felix, the ocean takes in most of the excess heat from greenhouse gases which leads to the tempuature rise in the ocean
Coral have a type of algae on them that they have a symbiotic relationship. The algae are what causes the corals colour. and their primary food source. This relationship can become stressed by ocean warming and/or pollution. If the coral has no algae it becomes white and loses its main food source, it is also more likely for it to catch a disease.