Dealt with this problem even with secure roll-up awnings even when stored under a carport type cover.
TRISODIUM PHOSPHATE (TSP) solutions are great for cleaning mold, mildew, grease, etc. 1/2 cup TSP to 2 gal. warm water, 2 quarts household bleach for ''heavy duty cleaning'' or
1 cup TSP to 3 quarts warm water, 1 quart household bleach for ''extremely heavy duty cleaning''.
Spray heavily or pour on canvas, allow to soak a moment, brush, repeat if necessary. Rinse. Dont permit to dry. An occasional wetting with a garden hose is fine. When clean rinse well for a few minutes after the stains and cleaner appear to be rinsed away.
As with any strong or caustic cleaning agents be sure to use eye/skin protection. ALSO, TSP can stain metal and bare/varnished/shellacked wood surfaces, but is OK for cleaning painted surfaces prior to painting.
TSP in available at Hardware, Home Center, Paint stores and such.
White Lighting degreaser found in cleaning/janitorial isles of most grocery/department stores may also work great.
Also, commercial and household mildew remover is usually helpful.
Boating/Marine supply stores carry commercial vinyl/canvas cleaning solutions of 5% Calcium Hypochlorite specifically for safely as quickly neutralizing mildew. It usually works in 1-2 minutes. I have also found them at Wal-Mart Auto, Sporting, and RV supply/cleaning departments.
Hope this helps.
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