Man what a mess! I know you all have seen this on houses before. This is what happens when there is an HVAC unit in the attic and the primary drain clogs up and the water overflows down into the drain pan(made of metal) underneath the unit. It then drains out the tube which is poked through the wall waaaaaay up high and allowed to just cascade down the side of the house leaving these ugly rust stains on the siding, the windows, the frames, the screens and whatever else. These can be next to impossible to remove, especially with just soap and water or some store bought “house wash”. Bleach won’t touch it, CLR won’t touch it and the only thing you will do with a pressure washer is damage the siding. You have to have a knowledge of chemicals/solvents and how to properly mix/dilute some pretty NASTY stuff and then safely apply it to the stain removing the stain but not harming the substrate or yourself! The stain pictured is actually on cement board siding, making it a little more difficult to remove. We were able to get this off and save the homeowner from having to paint and seal this whole side of his house. It saved him a lot of $$$ as well and he was very happy about that! My advice to anyone with this problem is to seek out a PRO that has experience with this type thing because of the chemicals and safety precautions that must be taken this is really not a DIY project. Good luck, I hope this helps someone out there.