Our marina in Stuart, FL is a paradise for mold/green/black stuff. In addition to the obvious Florida heat and humidity it is way up the South Fork of the St. Lucie River and has TONS of bamboo, palms, mango trees, etc growing all around the dock. Boats in the canal start turning green in 2 to 3 weeks if not constantly kept clean.
As such, I battle mold all the time, including on the dinghy cover, so I am watching this thread with interest and I am open to all ideas.
As for me, I haven't found anything so far that works as well as a bleach solution. I have tried commercial mold and mildew removers and had them do nothing at all. I've tried a vinegar solution, again with zero success, but recently read to try it at full strength, which I may do.
I've also tried the mold preventative sprays on the dinghy and cover with not much success. I agree that keeping them clean helps a lot.
We get a lot of mold on our light gray stamoid bimini. I wash the top of the bimini about once every month in the summer and two months in the winter (it's a PIA to do) in that much time it is close to black with mold and plant matter. I use a mixture of boat soap, water an bleach, about 1 cup of bleach in 3 gallons of water. Let it soak for 30 minutes to an hour, then wash off. I do it as the first part of my boat washing routine as the run off on to the rest of the boat is nasty.
The underside of the bimini also gets mold on it quite quickly, I hit it with a milder solution every time I wash the boat (once a week) then hose off and squeegee. That works pretty well.
I do the same process for our dinghy cover, I'm not sure what it is made of, it seems like sort of a cross between sunbrella and stamoid. It's an "over the counter" cover, not real expensive. I get two to three years out of them before they fall apart, then replace them. I'm sure the bleach shortens their lifespan, but I haven't found an alternative. I'm personally not a fan of sunbrella, though I'm sure it would last longer. I've priced having a custom cover made out of Stamoid, but I can buy 6 pre-made covers for the cost of a custom one, so I just treat them as sort of disposable.
I would LOVE to hear if someone has had success keeping the mold off of the dinghy tubes under the cover. I use a vent and the cover is open at the transom so it gets some air, but it is still a problem.
