Bleach is the answer.

Yes, use bleach. Diluted bleach kills mold dead, doesn’t leave residue, and won’t stain the plastic.

Mix one tablespoon of bleach per quart of water. Scrub the inside with a soft brush or sponge, let it sit for five minutes, then rinse thoroughly with fresh water. Let it air dry completely before storing with the lid open.

White vinegar works in a pinch but takes longer and smells. Skip the baking soda — it’s a mild abrasive but doesn’t kill spores. Also, do not mix bleach with vinegar or anything acidic; you’ll make chlorine gas. Just bleach and water.

Once it’s clean, store the cooler with the lid cracked or a couple of dryer sheets inside to keep the mustiness away.