Halimeda species are calcareous algae, meaning they will uptake calcium from the water and use it to grow larger although they never really get real large and will certainly not become the type of algae you would want to remove from your aquarium. They are real nice additions and I encourage everyone to try and add some to their reef tanks.
Two common varieties found are H. discoidea and H. opuntia. The former looks like an open piece of lettuce and is thus ribbed. The latter is characterised by leaflike extensions that grown from the main branch.
Both are usually found on sandy bottoms but H. discoidea can attach to hard bottoms such as rock too. Make sure you provide the rigth attachment means when you place them in your aquarium. Sand is best in both cases.
Halimeda grows well in environments with high calcium, medium light, availability of low amounts of PO4 (real low ones suffice) and low amounts of iron in the 0.05 ppm ranges. You may need to supplement the tank with iron if you wish them to grow better and be greener. You can read up on iron supplements in this SW library under TAT Product Related Articles.
Albert