Which term refers to the organisms that form coral?

The term that refers to the organisms that form coral is "coral polyp." Coral polyps are small, soft-bodied organisms that belong to the phylum Cnidaria. They are typically only a few millimeters in diameter and have a cylindrical shape with a central mouth surrounded by tentacles. Each polyp secretes calcium carbonate, which builds up to form the hard structure of coral reefs over time as many polyps grow and combine together.

Coral colonies are groups of these polyps that are genetically identical and live together, but it is the polyp itself that is the individual organism responsible for coral formation. Coral branches and coral reefs refer to the structure and ecosystem formed by multiple coral polyps, but they do not define the organism itself. Therefore, "coral polyp" is the most accurate term for the organisms that contribute to the formation of coral.