I am very maternal and when I see something that is wrong I tend to take care of it. I saw a guy at work who must have bumped into something chalky and white, it was on his dark trousers. I didn't even think about it, but I started to brush it off the back of his pants. I knew him, but when it hit me that I was brushing it off his back pocket I realized how weird it was. I would probably let someone know, I am like that.
No...they"re going to wonder wtf you were looking so cclosely at their butt.
I don't see what difference it makes if the person is cute or not.
If it's a girl, I might tell her. Depends on how noticeable it is and whether he/she can do anything about it.