Branching/Encrusting Fire Corals

Millepora alcicornis

Fire corals are colonies of hydroids that build structures like corals do, but are not corals. They are related to other hydroids like the Christmas Tree Hydroid. If touched can cause irritating sting and redness and an itchy heat sensation for several days. If severely contacted, may eventually cause skin to peal. See Things that Sting.
The growth patterns of this species (morphology) vary according to they type of environment. In high surge areas the colony may grown as expansive encrustations of large areas of rock or ship wrecks, or the tall, thick, flat-top blade colonies.  In less surge areas, the thinner finger patterns.  The age of the colony is suggested by how large the encrusting base.  For more information Go Here (external link)