Many homeowners that use front load washers have come to find out that the door gasket will begin to build up a moldy/mildew like black covering on it. Unfortunately most cleaners are not effective on removing this. The only solution that I have seen is actually replacing the door gasket (sometimes referred to as door boot). Going forward the best way to prevent this from happening is to wipe the door seal clean after every use and not allow any standing water to remain in the seal. Also try to leave the washer slightly ajar. They do sell Washer cleaners like affresh which can help clean the interior of the washer but the best solution would be prevention in the first place. If you are interested in having the door seal replaced you can call and we can give you a rough estimate over the phone with the model and serial number of the washer.