Scientific Name:   Halimeda sp.  (Halimeda discoidea shown)
Classification:        Algae
Common Names:   

Description:
Halimeda is a slow growing calcareous algae.  It has a holdfast (root) which is normally attached to the sandy substrate, but which may also attach to the live rock.  The body of the algae grows in a jointed plate fashion.

Good or Bad?:
Halimeda is probably the best macro algae to have in a reef tank.  It is slow growing, so it cannot easily become a nuisance.  It grows from a holdfast rather than runners, so it does not spread and take over the reef.  It also does not spontaneously combust as easily as Caulerpa, but the fronds will occasionally die and turn white due to the high percentage of calcium in the algae where as Caulerpa turn clear.  These actually contribute a great deal to the sand in many beach areas.

Notes: 
Halimeda requires fairly high calcium and Alkalinity levels similar to stony corals to survive.  Halimeda also prefers higher intensity lighting.