Bleach is your best bet for cooler mold.

Yes, you can clean it — but don’t mess around with just soap and water if it’s really taken hold.

Mix one tablespoon of bleach per quart of water. Scrub the inside with a soft sponge (avoid abrasive pads that’ll scratch plastic — mold loves scratches). Let the solution sit for 15 minutes, then rinse thoroughly. No bleach smell will remain if you rinse well.

Vinegar works for light mildew, but if you see black spots, go straight to bleach. The mold is already in the porous spots. You want to kill the roots, not just wipe the surface.

Dry the cooler completely with a towel, then leave the lid open for a day before storing. Moisture is what brought you here in the first place.