Camp Tech Eleganza Awning

I was wondering if anybody has any ideas as to why I’m getting white powdery streaks down both sides of my awning. This is my second year with this seasonal awning. The first year near the end of the season the white streaks were there but I managed to wash them off with warm water. When packing away we washed the roof of the awning and put it away perfectly dry. This year the white marks are really bad and I can’t wash them off. They are obviously coming off the roof after rain , they are even staining the side windows now. If you damp your hand and run it over the roof you get a white powdery substance. Definitely not coming of caravan roof as front widows would be affected. The awning is now 18 months old. Any ideas what this could be ? A few people on site have pointed it out to me and asked me what it is but I really haven’t a clue.