That’s water oxidation, It turned it yellow. It’s actually called “browning”
And it won’t remove with regular cleaning…
There’s a chem specifically designed for it, if you have a carpet cleaning supplier near you, I can instruct you what to buy
Buy this, Put it in a spray bottle, pre wet the awning and as it dries mist this on the stain and watch it disappear before your eyes, no scrubbing it’s going to disappear as it dries… But it has to be done in the sun 
