Bump

Use a bleach-based shower cleaner spray. Spray the area, let soak for a bit and then scrub with a brush. While you're at it, remove the detergent tray and look up inside. You might be shocked. Clean this area as well. Rinse well and then run a rinse and spin cycle. The rubber boot might be permanently stained.

To prevent this from happening in the future, use a towel or microfiber cloth to dry the rubber boot at the end of each wash day. Remove the detergent tray and dry this area as well. Leave the tray out and the door open when not in use. You should never have this issue again if you spend a few minutes dong this at the end of your wash day.