In UK most people will say "Good morning". You can say "Morning", but I believe "Good morning" is more popular. Conclusion: Use "Good morning".

You can just say 'morning', at least in Australia. It's very common here. Just saying 'morning' is more casual than 'good morning'. You'd be more likely to say 'morning' to people like friends or strangers than to professors or colleagues.

"Good morning" is the standard phrase. Like any casual form or piece of slang, it's best to wait until you hear other people using "morning" before you start using it.