Elephant skin coral
This encrusting coral is mostly found in deeper water, on reef walls and protected reef slopes and the symbiotic algae that lives inside it is adapted to low light.
Like most corals this coral has other creatures that live on it, on the ridges between the mouths of the coral there are tiny teeth like projections and in between these are barnacles. One species of barnacle lives on the top of the coral and a different species lives beneath it.
Like other corals this coral covers itself with mucous, not the bland stuff that comes out of our nose, but one full of sugar that is used to help trap food and share it among the creatures in the colony. The mucous also has bacteria in it that will defend the coral from harmful diseases and minerals that are used to build the colony.
Many of the fish you see feeding on coral (parrotfish, butterflyfish, wrasses and gobies) are actually like the kids in school picking their nose; they are eating snot.