Hi That stain is prob. rust from the wrong kind of hardwear used in the window install.
Go to www.dryvit.com they recondmend Sure Kleen products. I have tried some of them and some will work, some wont depends on the source of the rust. Also it will come right back after the 3rd rain. i have also thought the stains were caused by electostatis [the same thing that causes black streaks on gutters] and tried gutter zap and alum. brightner niether worked TEST TEST TEST then offer no garentee on them staying clean Sure Kleen is made in NJ. USE NO PRESSURE If you cant find a chem to clean it and wash it off with a garden hose then it cant be cleaned. If you do find a chem. please post it. thanks