Caulerpa verticillata

[sculpin]

Here's a Caulerpa species which is less commonly encountered than your everyday C. racemosa. C. verticillata grows with very thin (2 mm) rhizomes closely applied to the substrate with thin upright whorling branches usually less than an inch in height. Although attractive, it can be a major pest in an aquarium as it cannot be easily removed from rock work.

http://home.triad.rr.com/cookedberrys/Cverticillata.jpg