Just wet sand it with 2000 grit and buff it out.
easy to say, but doing any sort of buffing or sanding near an edge where the paint is thin could get you in trouble pretty quick.
Tru dat
I might just look for a good detailer to take care of it. I'm an amateur at best when it comes to that.
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Hit it with some Meguiars M105.
If that doesn't get rid of it, you're probably gonna have to get it wet sanded out by a pro.
Update:
The stain turned out to be a calcium/mineral deposit.
I soaked it with a 50/50 solution of distilled water / white vinegar and it came off almost instantly.