Because cynobacteria is composed of many different strains, its color can vary. Weather it is GHA or cynobacteria some of the cure is the same. While inorganic phosphate in the tank water can be removed with phosphate resin, organic phosphate in the substrate will not be affected with phosphate resin. Vacum the top 1" of your substrate removing macro biomass and detritus, which is the source of your phosphate.
Patrick