Acropora is the reef-building coral genus described as dominant in many Great Barrier Reef habitats.
Acropora includes branching, table-shaped, digitate, and other fast-growing corals. These forms can create complex three-dimensional structures that provide shelter and feeding surfaces for reef animals. Because they grow relatively quickly, Acropora species can be important contributors to reef recovery after disturbances, although they are also vulnerable to heat stress, storms, disease, and physical damage.
The genus contains many species, so Acropora is a genus name rather than the name of one coral species. Its members belong to the hard-coral order Scleractinia and build calcium-carbonate skeletons beneath their living tissue.
Porites is another major reef-building genus and can form massive colonies, so it is a plausible alternative. The question’s wording refers specifically to the genus identified as dominant in many habitats in descriptions of the Great Barrier Reef, not to every coral that contributes to reef construction.