Some older awnings get black marks on the plastic windows. The cause is the rubber peg ties being folded to the plastic. There is a chemical reaction between the rubber and the window plastic which results in black marks.
The short answer is that it is almost impossible to remove the marks as the chemicals from the rubber have migrated into the surface of the plastic. It happened to us on an old canvas awning (that's going back a bit!) and on a six berth frame tent. I never did find a way of removing the marks. In the end we gave up and lived with it.