You can try this:
Four Winns had this concoction that I have used with no issues (**This was on my Four Winns so I strongly recommend testing in a small patch first as your mileage may vary with this!!)
(1) teaspoon ammonia
1/4 cup Hydrogen Peroxide
3/4 cup distilled water
Rinse with water and dry....I always used a vinyl conditioner after this as well.
Might be surprised what it will do, especially if left out in the sun to cook a bit after application. Took a set black walnut stain right out of my white vinyl.
I agree though, that appears to be mold and a pretty good case of it too. IF it was a burn, you'd see remnants of it on your cover as well.