Mildew is a form of mold. It is very dangerous for all fabrics, especially quilts as it not only stains the quilt fabric but also eats away the fibers over time. You can remove these stains in a very simple way. Simply take the quilt outside and brush off the area infected with mildew so you won't breathe in the mildew spores. Then gently rub Ivory liquid soap or Orvus soap, or detergent on the stain and wash it off. This would remove the stain. You can also dry clean the stains if they are too old.