Corals are animals living in mineral houses! The minerals are deposited by the coral to form the antlers and other forms of the coral shape!

They are not plants! (From Wikipedia! https://en.wikipedia.org/wiki/Coral_reef )

Coral reefs are diverse underwater ecosystems held together by calcium carbonate structures secreted by corals. Coral reefs are built by colonies of tiny animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps belong to a group of animals known as Cnidaria, which also includes sea anemones and jellyfish. A differenza degli anemoni di mare, i coralli secernono carbonato duro esoscheletri che sostengono e proteggono i polipi del corallo. La maggior parte delle barriere coralline cresce meglio in acque calde, poco profonde, chiare, soleggiate e agitate.