From the information given, I'm assuming this was daytime in a populated area. (...) At night with no one else around? Then it might seem scarier. But that doesn't seem to be the case here.
Daytime, yes. Populated area, no: it's August, which means that most people are on vacation, city streets are empty and all shops are closed - bars, cafes, pharmacies, everything. I couldn't see anybody else on the street but me and Annoying Guy.
Women are usually advised to walk in a café or shop if they are being followed, but I wouldn't have been able to since there were none open in the immediate vicinity.
Also, PP who thought I was overrreacting to a simple "hello" - please keep in mind I'm not American. Over here we don't greet random people on the street. It's just not part of the culture.
The funny thing is, I was greeted on the street the very same morning this happened - but the man who did it probably knows me by sight (I'm both short-sighted and horrible at face recognition so I can't quite say the same) and though I didn't recognize him, I was not scared because he just said hello and kept going on his way, without slowing down or changing direction.
I didn't feel scared by Mr. Green T-shirt either, perhaps because I istinctively knew he might be worth watching out for before he even opened his mouth - which is why I managed not to answer his bait-greeting.
Also, he didn't just greet me: he deliberately
changed direction in order to follow me. I'd say that's telling.