Molds are plants. Plants need a food source and moisture. Maybe re-positioning the grill during storage to a place where it can get some sunlight. Use a bleach and water solution on the mold - then leave the grill open for the sun to dry it out. Something else that may help...when you are finished with you cook - after removing all foods, turn the heat up all of the way for 15-30 minutes. This will help to burn off any residues from foods or at least "de-nature" them by burning them until they are carbon.
Hope that helps.
T