Ordinarily, coral are brightly colored because of algae, called zooxanthellae, that live in their tissue. It's a symbiotic relationship. The coral provide the algae shelter and compounds that they need for photosynthesis, and in exchange the algae help the coral remove its waste, and provide it with oxygen and, most critically, nutrients in the form of glucose, glycerol, and amino acids—the byproducts of photosynthesis. These help coral grow, albeit slowly. Depending on the type of coral, coral grows as little as a .1 inches a year, meaning a dead reef will not bounce back within any of our lifetimes.