I had the same problem on the lower side of my vinyl cockpit seat cushions. Try an automotive product called Bleache White. It comes in a spray bottle and was developed for white wall tires. I spray the stuff directly on the mold/mildew (works best on a dry surface). Use a scrub brush to work the bleache white in and then wash it off with a hose. It works great on plastic furnature, fenders, dirty line, and bird droppings on the deck. I have tried it on teak with good results.