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.