It's not rude, but it just sounds weird. "Guy" is also a male name. I would say my name is not Guy in reply. In fact, if you do call me "guy" this would tell me that you don't understand English well. "Guys" is OK for addressing a group (of males). Using "guy" in the singular form only makes sense if you talk about "him" ("that guy there"). For greeting a stranger, just say "good morning". That's it. "Hey, morning" is used if you know the person. If you say that to a stranger, they'll likely say, "Do I know you?" Not a good introduction. On a side note, we Australians do drop "mate" into our conversations often, so it is actually acceptable to say "Good morning, mate" to a stranger. But only in Australia.

It isn't rude, but it's informal, so I would reserve it for people of the same general age and status. You could say it to a friend, co-worker, or stranger who is around your age; I would not say it to a boss, an elderly man or a customer.

It's better to say... "Hey! Good morning, man." If you say it sincerely with a smile, the man/guy would probably say "good morning" back and think you're friendly. However, if you look strange and suspicious, that would probably be awkward. You use "man" like "dude". You can say: -What's up, man? -What's up, dude? But NOT.. What's up, guy?

Just stick to Good morning when you meet a stranger.