Or I found this online too
Follow these steps to remove mildew stains from Acrylic Fabric, Cotton, Linen, Modacrylic, Nylon, Olefin, Polyester and Spandex. Most mildew stains can be removed during regular laundering if they are moistened beforehand.
If a stain remains:
Test fabric for colorfastness.
If color doesn't change, cover stain with a paste of lemon juice and salt.
On cotton and linen, make a paste from an oxygen bleach, water, and a few drops of ammonia.
Let paste cover stain for 15 to 30 minutes.
Flush thoroughly with water and launder again.