i find it hard to believe that that big chunk passed through a ball lock valve. if anything, maybe your air lines or regulator are infected now, and they are contaminating your keg??? (the mold grew inside the keg)
the way i keg is as follows and does not give much of a chance for mold growth: i ferment, then rack into my keg, pressurize the keg, and immediately put it into my kegerator to carbonate. since the clean kegged beer is going right into 40 degrees i think it prevents any mold that can grow in beer from growing.