Try soaking it in vinegar then do laundry spot cleaning.
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.
Try WD 40 it's great stuff
White vinegar, baking soda or borax all work well.
Our DIY mold cleaner also works for fabric. Test a small area first: http://chemistrycachet.com/the-best-way-to-remove-mold-mildew/