I haven't found much on the internet about how to successfully get mold & mildew stains from clothing that has gotten wet. In the past, I have thrown away things with sentimental value because they got ruined in storage... but I stumbled on something that actually worked really well.

Before you try washing the fabric with anything, lay it out and pour undiluted Clorox 2 liquid directly on the mold/mildew. It will begin eating away at the stain and foam up white. The first time I did this, I washed it right away but residual stains were left in the fabric that were hard to remove AFTER it was washed. So, I tried it again (on a different fabric/new stain) and let it sit overnight and washed in hot water with detergent and a half a cup of Borax. It came out without a trace of the stain. This works well for color fabrics that cannot have bleach used on them as soaking in the machine with detergent & bleach overnight will kill anything on white fabric. (but using cheap bleach can cause yellowing)

I just wanted to share the tip because I haven't been able to find anything that worked as well as this.