The iron content of the disc is turning rusty, and there'll be some metals in the friction material which might corrode in their own little way too.
It's only a tiny bit of surface rust and won't be a problem until it gets to the state where you can't free it, no matter how hard you try! Then you might have to take the callipers off (leaving the pads stuck to the discs) and then pry the pads off with your best serving spoon - but that's worst case scenario.
If it annoys you (more than I do), then dry it off as much as possible before putting bike to bed.
Threaded Mode