Possibly the nutrients were established early in the pond history and are recycling. Options to consider add enough bottom rooting fish to keep the pond turbid and use dye to make it look aesthetic ('lipstick on a pig'); use an alum treatment to bind and reduce the phosphorus; If the pond is old, drain it and have the nutrient sediments removed. If you were not in MO, tilapia are a good algae consumer especially for swimming ponds.
