Carbon can't remove enough nutrients from the water to help with algal or cyanobacterial problems.
0.03 ppm is not low for phosphate. It's borderline, and could be a problem for stony corals. Its presence indicates the tank has a nutrient problem, most likely from overfeeding. Other nutrient sources can include topoff water and the live rock.
This article might help:
http://reefkeeping.com/issues/2006-09/rhf/index.php