Healthy corals are brightly colored because they healthy corals are brightly colored because they secrete colorful pigments to protect themselves from ultraviolet light secrete colorful pigments to attract mates host symbionts with colorful photosynthetic pigments build their skeletons from colorful minerals

The answer would be: host symbionts with colorful photosynthetic pigments

Coral can host symbionts that will help in their growth. There are algae that could live on the coral surface, giving them some of the energy from photosynthesis and help in calcification while the coral protects the algae. The symbiosis will be formed when nearby algae attach to the corals. A stress on the coral can cause the algae to eject the algae.