Mold stains can be difficult to impossible to remove from fabrics, depending on the age and extent of the stain.  However, mild stains that have not penetrated fabric are worth trying to remove and often do come out or at least fade with some effort. First take the bag outside and brush off the mold with a stiff brush. If you cannot take the item outside, use a stiff brush and brush the item over a piece of newspaper. Then, roll up the paper and throw it away. Try to remove the stain with the following procedure:1-Pour 8 ounces of  water, two teaspoons of salt and two teaspoons of lemon juice into a spray bottle and tighten the nozzle; shake the bottle gently until the salt is dissolved;  2-Spray the cleaner onto the canvas until it is visibly saturated; wait five minutes, then scrub the mildewed area with a brush; rinse the cleaner off with clear water; 3- Inspect the canvas to see if the mildew stain was removed by the first cleaning; if the stain is still there, mix the other two teaspoons of lemon juice and salt in the bowl to make a paste; apply the paste with the brush and scrub the paste into the canvas fibers until the stain is gone; 4-Rinse the paste off of the canvas with clear water and let the canvas air dry in a sunny, well-ventilated area.