mold and mildew

initial quality of vinyl goes a long way to determine longevity -cheap plasticizers,cheap stabilizers and cheap coloring= cheaper costs shorter life
we top coat our vinyl roofing membrane with an acrylic lacquer, use the most expensive high molecular weight long chain linear plasticizers,use TiO2 as pigment instead of CaCO3 costs more to make -sells for more than any other membrane on market but lasts for 30+years plant has been running 24/7 for 4months and still 3wks behind on orders
we tried adding biocides, fungicides and other treatments to inhibit growth after damned enviro wackanuts told us to get the lead (Pb) out
we still have 25yr old roofs that look better than others 3 yr old products
not to encourage breaking open a battery and using like a clay bar But mold would run away - far away -just remember to wash before and after you use restroom

i work for a pvc membrane manufacturer and have done some work with numerous products for prevention and removal of contamination
there is no magic potion and what may appear as snakeoil for 1 application may actually work well for another
vinyl needs external plastification to achieve and maintain flexibility, plasticizers are added in oil form to raw materials. they will leach out from uv degradation and time and process is accelerated by temp or contact with incompatible surfaces, some types of foam eps,xps asphalt etc. dirt and organic (mold) stick to oil that rises to surface,aggressive removal srcubbing or use of some solvents will clean but damage- plasticizer loss leads to embrittlement, dryrot, crazing
chlorinated products work best for removal but do not saturate -blot with cloth and bristle brush rinse well allow to dry and then use damp chek or silica packs if you are not able to maintain ventilated storage
some have had luck with "303" protectant

Don't put Armour-All on your vinyl and then put the cover on, it promotes the growth of mildew.

I had similar black spots & had to scrape them off with a sharp knife. The mildew remover wouldn't cut it.   I am sure I took the vinyl's clear coat off too. The mildew eats right into the vinyl.

If you have very much of this, it is time for new skins. The mildew will come back quickly.

We utilize 3M marine mildew stain remover part # 09067.

"Beware" This product contains bleach. Bleach can and will destroy the threds (stitching) that holds your vinyl together. We utilize the product only on the vinyl and utilize a soft bristle brush followed by copious amounts of water to rinse away any residue. Follow up with 3M vinyl cleaner and conditioner and restorer part #16904. Give your boat enough time to dry completely out cover and store with good ventilation.