Power Compacts are generally quite sufficient If the tank is heavily fed, that will generally provide all the nutrient input needed. The only extra I find that helps is iron suplementation. Nitrogen and Phosophorus only need additional inputs if the tank is lightly stocked and sparsely fed.
Strong light, adequate flow, and nutrients to grow most macros. Some uptake nutrients via the water column and others use roots. You may need to target specific needs for different species.
The turning white is, I believe, a typical symptom of not enough light.
I think that Halimeda is a light demanding species. When I was in Jamaica the place that I saw it the most was in this shallow tidal lagoon that went right into a mangrove forest.
Halimeda is also calcareous so it needs to uptake Ca so that might have been the problem as well.
Tanto91: NO lighting on a 55 is probably not enough light. I have a 20g fuge that has 65watts of PC lighting. The advantage is that itÃƒÂ¢Ã¢"šÂ¬Ã¢"žÂ¢s more intense lighting (than NO) on a shorter (height) tank.