I knew I read somewhere where they can eat up some trates and somefish will pick at them. Definately will eat up the calcium though.
"The Halimeda Plant is a green calcareous algae found worldwide, and used as decorative plants in the marine aquarium. The irregular oval segments of Halimeda appear as several small green coins glued from end-to-end, forming a chain. For this reason, Halimeda may also be called the Money Plant. Calcareous algae deposit calcium carbonate in their tissues. This is a beautiful plant that will add a variety of color and form to your aquarium whether it is a reef or fish only system.
Halimeda Plants are generally hardy in the right conditions, and few fish will feed on them. In addition to light, nutrients and other trace elements, they require sufficient calcium to grow. A pH of 8.1 to 8.4 is ideal for the proper growth of these plants. They are not as tolerant of high nitrate or phosphate levels compared to some other algae. They also do not tolerate extensive pruning.
The Halimeda Plant is shipped as a bunch, and is not attached to any substrate including live rock. To position this plant in the aquarium, simply place or tie the clump into the rockwork. Once attached, the tie may be removed."