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.