I have a houseboat with carpet on the decks. It gets some really bad mildew stains. My wife started putting straight white vinegar on the stains and after sitting a bit hoses the vinegar off and the stains are gone. In our case we have to deal with a bit of vinegar smell for a bit but it might not be an issue with a cap. Maybe just rinse it again if the vinegar smell lingers.
Ok, time for the douche comments. 😜