here are a couple ways I would try on some of the same fabric before using on the quilt.
Lemon Juice and a tooth brush to open the pores of the fabric, with dial bath soap then rinse with the water and pour lemon juice over freshly cleaned area a second time lay in direct sunlight until dry. then wash in machine . This works for cotton. without discoloration on pale blue
I had a mold stain on the back of a yellow Melton coat (wool). I used a combination of Melamagic and Sol-U-Mel, both products of the Melaleuca company, (available on line from the company). I used about two tablespoons of each, diluted with an equal amount of water and poured the solution over the stain. It disappeared before my eyes. Because it was so fast I worried that it might hurt the cloth so I rinsed it immediately and well but no harm done.
Hope this will help :?