The species Halimeda incrassata is a bushy plant of variable appearance, growing to 25 centimeters, or 10 inches high, though usually it is shorter. It is a rather cosmopolitan tropical seaweed, one that is abundant on sandy seabeds. The formal vernacular name of H. incrassata is the three finger leaf alga, but it is rarely circulated by aquarists. It is one of the cactus algae or sea cactuses, of the genus Halimeda, which are also known as coin seaweeds.
These seaweeds give the appearance of small, hard plates, that look strung together; this is outwardly somewhat similar, to a prickly pear cactus (Opuntia sp.). Of course, the two clades are not related; for whereas desert cacti are obviously land plants, Halimeda sp. are, in fact, siphonous green algae, and their resemblance to certain cactuses is very superficial.
The reason why each segment is hard, is because they are impregnated with calcium carbonate, unlike the connecting tissue between them; as with the limey skeletons of the stony corals, those of Halimeda sp. degrade into sand, and Halimeda are so important in shallow marine ecosystems, that they produce vast quantities of 'Halimeda sand'; and this voluminous material contributes to the formation of atolls.
Halimeda it is one of the 'coralline algae' by some definitions - in such a sense, they are a non-phylogenetic group of macroscopic marine plants, all possessing hard skeletons. Cactus algae are unrelated altogether to the corallinale clade - the encrusting red and purple coralline algae that many reefers love; among green algae they are related to genera such as Chlorodesmis, Penicillus, Udotea, and Cauleroa.
Some Halimeda species form thickets in lagoons; but although they are obvious and easy to identify as Halimeda, there is uncertainty and confusion about the number of species, and their demarcation. To the reef aquarists, what matters primarily is that some Halimeda sp. - such as H. incrassata - are associated with sand or coral rubble.
Whereas other species in the same genus, grow attached to hard substrates, in locations with conditions of higher flow, but in relatively sheltered microhabitats. (Halimeda should not be exposed to higher than moderate currents.) Yet other members of the genus grow according to a sprawling habit. But the lagoon-associated H. incrassata is one of the sand growing species.
H. incrassata is a tropical seaweed that grows best at a temperature between 26 and 28 degrees centigrade, a little warmer than are most marine aquariums. Neither the light nor the water flow need to be strong, because this plant grows in turbid environments that are sheltered from the most severe wave action.
Some sources insist that Halimeda sp. are rather immune to the attention of herbivores. In truth, they suffer a lot of grazing, in the wild; it is just that their calcification deters ill equipped species from biting them. Around the reef, different herbivores consume different plants, either out of need or preference, just as grazers and browsers do on land.
Fish that graze on well established Halimeda often tend to have narrow diets, indicating they tend to be specialists, rather than generalists. Parrotfish and scarids bite at Halimeda much more often than do acanthurids or siganids, and these seaweeds are not preferred as food by either turf grazers, or browsers of fleshy algae.
It has been confirmed that a number of Siganus species are outright averse to eating cactus algae because it is calcified; these include S. argenteus, S. caniculata, S. radians and S. spinus. Similarly most acanthurids such as 'Acanthurus' triostegus are not Halimeda consumers at all, or they appear to consume it only incidentally or when it is newly settled, as do Ctenochaetus strigatus.
Proper feeding on Halimeda appears limited to some of the acanthurids possessing gizzard-like stomachs, such as 'Acanthurus' blochi, 'A'. cyanocheilus, and 'A.' olivaceous. These are not the species most popular among aquarists. So it is true that Halimeda will not be grazed by tangs, but it is only a rule of thumb.
Other Halimeda species have different substrate needs. H. tuna for example, grows upon hard bottoms in lagoons and deeper, sheltered locations down the reef slope. H. opuntia grows inside the grooves, depressions and cracks in rocks, and between coral heads in moderately protected parts of the reef. Such species have also been traded as ornamental plants for marine aquariums, and also introduced accidentally as benign and helpful 'hitchhikers'.