Ok,..I see no signature of any kind,..not sure if you're a business or a handyman or a broke college student or what,..but if this is a service you're wanting to start,..you really need to get a better game plan than you displayed here. So,..I'm gonna address this as if you are doing this as a service.
Definitely oxidation. On a positive note, to me the paint looks to be in good enough condition to tie it all together and have a decent result. Yes,..it will take some effort,..and these dark colors are the worst at getting correct. You need a cleaner like Purple Power or something similar,..apply it very evenly with a pump sprayer,..and brush it all. The PP will make the oxidation loosen up and run off alot on its own,..especially if it's nice and warm or even hot out. Start with a mix of 50/50 PP to water. But that siding looks like it will take full strength,..and ALWAYS do a test spot to make sure the paint will handle the cleaning method. Worst case scenario is you'll have to clean it the way I mentioned,..and you will still end up with a customer who wants a paint job.
*That looks terrible,..When washing anything,..you have to use detergents for the job at hand. Doing any exterior cleaning without proper detergents is NOT the professional way. This really is what gives this service a bad reputation, and many people who are potential customers look at this kind of thing and think we're all the same.
**A skill that is REALLY needed in this business isn't only how to clean,..but how to evaluate. And that job needed better evaluation.
Jeff