Your thread is well named.
Aluminum siding falls into two categories for us.
1. If it has been repainted using proper prep and good quality paint, then our low pressure detergent wash usually cleans it up very nicely.
2. If it has a "very thin coat of paint on it", then we call that a paint prep. Our low pressure detergent wash will remove mold, mildew, dirt, etc, and make the painters job easier.
Aluminium siding that shows a "very thin coat" usually means that the paint is thouroughly oxidized and surely a good bit of that oxidation is going to be removed when washed. It is expected that the washed siding will be blotchy and even less attractive than before, but it will be clean.
Good luck, and post pictures if you have them.