Google gave several solutions.
1. Oxy-Clean
2. A mixture of salt, vinegar, and water should remove mildew stains on most fabrics. Use up to full-strength vinegar if mildew is extensive.
3. To remove mildew from upholstery and other fabrics, soak a sponge in a solution of 1/2 cup borax dissolved in 2 cups hot water, and rub it into the affected areas. Let it soak in for several hours until the stain disappears, then rinse well. To remove mildew from clothing, soak it in a solution of 2 cups borax in 2 quarts (2 liters) water.
Hope one of these works!