Spray the artwork lightly on both the front and back with Lysol. Do not use Lysol in a bottle.
Allow the spray to dry and test a small patch to see if the mold is powdery and dry. If it isn't, repeat the Lysol spray treatment until the mold is dormant.
Take the painting outdoors and place it on a clean surface.
Use a soft brush to gently remove the dormant mold. Do not grind the mold into the canvas or paper.
Check for mold under the canvas stretchers, frame and mat. If it's present, use the soft brush to remove it.
Try an alternative method if Lysol doesn't work. Expose the art to bright sunlight for an hour and a half. Check frequently to make sure the sun isn't damaging the art. Or, place the art in a freezer that is not frost free and that doesn't cycle on and off. This method doesn't kill the mold. It simply arrests its development.