Halimeda is a calciferous algae which requires good levels of calcium and magnesium to proliferate. If you haven't read these articles on water chemistry, here they are (again):
Aquarium System Volume Calculator:
http://home.comcast.net/~jdieck1/volcalc.html
The Reef Chemistry Calculator:
http://home.comcast.net/~jdieck1/chem_calc3.html
Reef Aquarium Water Parameters:
http://reefkeeping.com/issues/2004-05/rhf/index.htm
Magnesium in Reef Aquaria:
http://www.advancedaquarist.com/issues/oct2003/chem.htm
Solving Calcium and Alkalinity Problems:
http://www.advancedaquarist.com/issues/nov2002/chem.htm