This is a colony of soft coral polyps known as zoanthids. They are more commonly known as a button coral, because their polyps resemble buttons. Although they look like plants, corals are actually animals. The green color comes from a species of photosynthetic algae known as zooxanthellae. This algae uses sunlight to produce food for itself and for the coral polyps. Button polyps are popular among reef aquarium hobbyists because they are easy to keep.