Description:
Caulerpa is a fast growing attractive algae of which there are many variations with leaves that look like ferns, grapes, etc.  Not surprisingly, the different types are named after the look of their leaves.  All grow rapidly by sending out runners which are held in place by root like holdfasts.

Good or Bad?:
Caulerpa is generally considered good when it stays where it is suppose too, but it can easily become a weed that can be difficult to eradicate in the reef tank.  If you introduce Caulerpa into your main tank, either on purpose or accident, keep a close eye on it and prune heavily to keep it in check.

Notes: 
Having Caulerpa growing in the reef tank was once considered a sign of success.  Today, it is more often considered a sign of trouble as the Caulerpa can quickly spread and become a weed.  Caulerpa is usually not kept in the display tank anymore unless a lagoon tank is the primary focus.  Most Caulerpa is maintained in filters or refugia where it can be confined.  When part of it is harvested from the these systems, it is acting as a nutrient export mechanism.  In other words, the nitrates and other compounds the algae absorbs as it grows is removed from the system when a portion of the algae is removed.

Some species, especially Grape C. can go sexual and disintegrate overnight adding substantial biological load to the tank.  This can be minimized by heavy pruning which seems to retard this tendency.  There is also some evidence that keeping the lighting on 24/7 will prevent this from happening.  This is obviously only an option when the algae is housed in a refugia or sump.  Herbivorous fish like tangs will sometimes help to keep the Caulerpa in check.       

Caulerpa growth seems to be best under medium to high intensity, low color temperature lighting.  Inexpensive Mercury Vapor or outdoor PC lighting works well when trying to maximize the growth of Caulerpa for filters or refugia.  It will even grow vigorously under normal incandescent lighting.