Clay bar is working, but it's not easy. The previous owner must've never waxed this thing because whatever got on here is really sticking.
@shortcake I took a black car to a different-than-my-usual carwash once because out needed washing and there was an hour to spare. It came out looking shiny as hell, but whatever they used to achieve that (a) was super tough, but (b) picked up mineral stains similar to what you're seeing, and they were a pain in the ass to remove.
Toot.Style is a private instance, meant for just a handful of users. I created it mostly to have an instance for myself that I could customize however I'd like (can you tell I like pink?).