This is probably Halimeda sp.or Udotea sp. These two macro algae use calcium to form a skeleton of sorts. They are relatively slow growing and require high calcium levels in order to thrive. Both are good in a reef and often give the tank a much more natural appearance as reefs are often composed of 80% algal matter. It too removes nutrients from the water and can be harvested as a form of nutrient export. The dead "skeletons" of Halimeda sp. are also a primary constituent of calcareous sand in reef areas.