The brown, bumpy disk is a large 6" diameter mushroom anemone in the reef aquarium. Though mushroom anemones are capable of capturing and ingesting food, they get most of their nutrition from the sugars produced by symbiotic algae (zooxanthellae) that live within their cells. These same algae also form similar symbiotic relationships with nearly all of the other anemones and corals in our aquaria, and also with the giant clams.
The green plant in the foreground is Halimeda, a genus of green algae that manufactures calcium carbonate (limestone) in its tissues as it grows. This makes Halimeda unpalatable for most herbivores, but the white bite marks visible in this picture are evidence of grazing by the blue tang (Paracanthus hepatus) in this aquarium.
In some areas of the Florida Keys, most of the white sand is actually the remains of certain species of Halimeda that grow in the shallow coastal waters.