I just got a new 2018 GLC 43 in May and noticed that all of the window trim on doors is discolored (see picture). I haven't treated the trim with anything, it came with the car. Any ideas what it is caused by?
*If I rub the trim, the discoloration disappears but not sure if it stays black. I have to test that.

Kinda looks like something got on it. Did u run through a brushless car wash or have you washed the car with anything abnormal? I'd probably try some good car wash soap and then a good rubber/trim cleaner to see if that works. If not, I'd beg* the dealer to help out.
*Dont think they will cover under warranty, but you never know.

I thought I saw something in the manual about avoiding particular types of car washes like maybe the touchless ones? I don't remember the exact details.

I'm doing 100% mobile detailing on this car. Its expensive, but I'm getting WAY better results than I was going to the local carwash

Its from car washes. I have had the same thing, if you wipe it goes away.

So I thought the same thing...I bought my car before they could take the shipping wrap off of it and had L1 paint correction, XPEL and Modesta applied to the car. After the first wash with DI water, the chrome trim fogged up and could not be polished off. I took it back to the dealer and they tried to polish it and had similar lack of success. They have ordered new trim for me and have given me a $1000 accommodation that I applied to my PPF/Modesta costs.

It would not hurt to take the car in and talk to your service manager. I was a particular PITA when it came to this...