Moldex Deep Stain Remover to the rescue.
It is a concentrated, multisurface cleaner that is packed with Oxygen-Power that gets rid of mold and mildew stains. It was formulated to quickly remove stubborn, embedded mildew stains that tough mold and mildew can leave behind after cleaning. Moldex Deep Stain Remover does not contain harmful bleach, ammonia, or other harsh chemicals. It is nontoxic, has little odor, and will not harm plants or fabrics like traditional cleaners.
Mildew stain removal from: Patio Furniture, Awnings, Shower Curtains, Walls, Tile, Grout, Plastic, Previously painted surfaces, Basements, Window frames, Sealed Wood, Tarps, Tents, Shingles, Roofs, Siding, Asphalt, Concrete, Foundations, Brick, Masonry, Walkways and Retaining Walls.