I don't think that I would find that too terribly creepy as long as you weren't creepy about it :D But seriously some(most) girls/women need to be told that they are pretty, you know with the world and society's focus about how women look, act and who they are in general, that can cause self esteem problems and negative self view points. So I don't know, no guy, except my boyfriend, has ever told me that so that could go both ways for guys and women. nikki minaj hit the nail on the head when she said," And I say girls are beautiful and girls are sexy and they need to be told that..."
"I just wanted to let you know you look very beautiful today, see you around"..and then you leave. lol. no gawking or hanging around waiting for her to fall into your arms or anything, simple and casual. bump into her, "oh ..wow..you look really nice today..". be simple but charming.
Every girl likes a nice, genuine comment no matter who says it.
I wouldn't find that creepy. At school, random guys in my class will call me pretty, not to say I don't know them, but they are truly random lol. I take it as a compliment, definitely not creepy. But don't go over the top when complimenting her and don't over exaggerate it.
Yeah, they might find that creepy. Try just talking to people like that when you see them out in public. Talk to everyone around you if you can so you can get an idea of how to socialize with complete strangers. But just telling a random girl that they are pretty won't do much, if anything for her.