The mold doesn’t grow on the awning fabric but rather on accumulated dirt and dust. There aren’t any products that I know of to prevent mold growth. Best prevention is to rinse the awning occasionally, to remove the dirt and dust you can’t really see, then leave the awning open to completely dry before rolling it up. (I have the same problem with my awning which is white on the bottom)
No magic bullet but I hope this is helpful to you.
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