Mix together a solution of 3/4 cup bleach and 1 gallon of water.

Soak paper towels or an old t-shirt in the bleach + water solution.

Then cover the dirty parts of the rubber ring on your washing machine with the paper towels soaked in the solution. Use a binder clip to keep the towels in place if necessary.

Let it set for 3-4 hours and then remove, and surprise all the dirt and mold growth will be gone.

Lastly put 1 cup of bleach in your machine and run it on the clean washer setting.