Certainly, I can't see what you're referring to so w/that in mind, I have to ask. Could it be chalking that has run off of your aluminum capping? The reason I say this is, well, I don't know what to use to remove EITHER but if there are 2 different types of 'rinses', one may actually make-permanent the other! For some reason, TSP comes to mind.
Go to your local Home Depot or Lowe's or whatever and look into TSP. Read the box and ask some questions. If it's the solution (not to coin a pun), buy some gloves and eye protection. Wear long sleeves and pants. Start from the bottom and work your way up. (NOT from the top down because, you'll 'streak' the walls.) Rinse the already-cleaned area(s) as you work your way up. Don't be alarmed that whatever the solution is you may end up doing the ENTIER wall! If you don't, well, you'll certainly know the area(s) you cleaned. I'd stay away from a pressure washer or use one on low power to rinse. A garden house should be sufficient.
Others may have more to offer. My best to ya and hope this helps.