What is causing this damage to the window? Mold? Age?

If the rest of the apartment is like that too (and it seems so according to your other questions), I would simply start with a complete renovation.

Yes, it’s starting to look like a complete renovation to me as well.

What kind of apartment is this? Dirty sockets and disgusting windows :eek:. You will probably have to tear everything out once.... judging by the condition of the things shown.

No, those are (many?) old layers of paint on a wooden window.
Tip for you as a beginner ;), if the window has a "Regenschiene," it is "usually" not a plastic window

To be honest, I don’t always bother to open all the attachments that need to be viewed in two steps.

If the rest of the apartment is like that too (and it seems so according to your other questions), I would simply start with a complete renovation.

A property is often poorly maintained in all aspects. Scrap dealers don't appraise cars "representatively" by the condition of the oil without reason.