Millepora alcicornis is not a true coral in class Anthozoa but is in class Hydrozoa, and is more closely related to jellyfish than stony corals. Because of the variability in growth habit that this coral exhibits, it has been the subject of much confusion as to its taxonomy, being described under a number of different names from different localities. In 1898, Hickson decided that the variations in morphology were due to environmental factors and that Millepora alcicornis was the valid name for all these species. This conclusion has since been questioned.
The species was first described by Linnaeus in 1758, but his type locality is unknown. The scientific name comes from the Latin with Millepora meaning "thousand-pored" and alcicornis meaning "elk-horned". It seems likely that the type locality is in fact the West Indies. Explaining this in 1941 Crossland wrote "I cannot resist the remark that the one thing quite certain about the many forms of Millepora is that none of them have any resemblance to an elk's horn, except perhaps that from the West Indies".
The morphology of Millepora alcicornis is very variable. Most colonies probably start as encrusting forms and adopt a branching structure as they grow. The encrustations can become established on a variety of structures, not only on coral reefs and rocks but also on dead corals and the hulls of wrecked ships. Later development is in the form of plates or blades in habitats with much water movement such as the surf-pounded outer edges of reefs. In calmer waters, such as in deep lagoons or more sheltered parts of the reef, a more upright, leafy or branched structure develops which can grow to 50 centimetres (20 in) tall. The habit of growth is also influenced by the inclination of the surface on which the fire coral grows. On vertical surfaces, the encrusting bases are larger with longer perimeters and the density of branching is lower than it is on horizontal surfaces. The cylindrical branches usually grow in a single plane and span a range of hues from brown to pale, cream-like yellow, while branch tips are white.
Embedded in the calcareous skeleton are numerous microscopic polyps. They are connected internally by a system of canals and are concealed behind pores in the skeleton, the surface of which is smooth and lacks the corallites of true stony corals. The polyps have specialist functions, the gastrozooids processing and digesting the food caught by the dactylzooids which are grouped around them. The gastrozooids are small and plump and extend four to six tentacle stubs through their pores but are otherwise invisible. The dactylzooids have hairlike tentacles covered in cnidoblasts. Stings from the cnidocysts immobilize an item of prey and the tentacles thrust it through the mouth of an adjacent gastrozooid, from where it passes into the stomach for digestion. The polyps also extrude the coenosteum, the calcareous material of which the skeleton is composed. The coenosteum contains certain symbiotic microalgae called zooxanthellae. These are photosynthetic organisms which provide their hosts with energy and in return benefit from a protective environment in a well lit position. About 75% of the fire coral's energy requirements are provided by the zooxanthellae.
Millepora alcicornis is found in the Caribbean Sea, the Gulf of Mexico, Florida, the Cape Verde Islands and along the coast of Central and South America as far south as Brazil. It has also been found in Bermuda, but the morphology at that location is so different from that in the rest of its range that it may be a distinct species. It grows at depths of up to 40 metres (130 ft) and is the only fire coral that often grows at depths greater than 10 metres (33 ft).