Yeah my guess is mental illness, someone off their meds, having a bad time of it. I realize it's probably unnerving, but you are almost certainly as safe as you ever are. That's a very public area and so you are probably safer there than a quieter street even if it seems like it may be the opposite. Try not to judge the people around the Green too harshly. Most of them are just there waiting for a bus and don't have a choice but to hang out in the area, but the bus stop also attracts a small percentage of people with mental health issues (or drug dependency issues) who have no where else to go.

Keep alert, but also don't look or act "sketched out" because people will understandably take offense to that. If something DOES start to go down, I feel like strangers step in to help where they can more than you might expect.