There are awning cleaners available that won't damage the fabric but you may need to waterproof it afterwards. I suspect there will be a mark left made by the mould staining. If so try a little lemon juice on the mark. Leave it for 20 mins and wash off. If this removes the mark or lightens it make up a solution of citric acid and use that sparingly. Citric acid is available from home brew shops and is the acid found in lemon juice. Treat it with respect and test a lttle bit of the fabric to make sure it does not damage. Keep it away from children and wear goggles.
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