With nitrate and po4 undetectable, your water might be a little too clean. Leathers like a little organics in the water. Try feeding it and see if it perks up. I would try something like phytofeast along with something like DT's oyster eggs.

I've seen plenty of leathers like that which have done nicely. Although I don't know how green it will get. It seems people are calling everything a Palau Green. For comparison here's a pic of a guy's Palua Green Nepthea (which is probably actually a sinularia)...
http://www.utahreefs.com/ReefTour200...s/DSC01261.htm