Why is it an evolutionary advantage for the coral polyps to only come out at night?

In summary, the advantages of extending your polyps at night are:

be exposed to less competition

be protected from predators

Explanation:

Corals generally feed at night by extending their polyps It is an adventage because they are protecting the polyp from browsing fish such as many butterfly fish that prey on corals.  

the element that provides strength to the exoskeletons of clams and oysters is phosphorus. this is so because calcium carbonate is the main component in the shells.

explanation:

producers: apple trees, potatoes, carrots, grains, acorns, cabbage, grass, corn, lettuce

consumers: deer, mice, rabbits, chickens, squirrels, horses, worms, cows

predators: eagles, frogs, wolves, owls