He says 'you alright xenon17' and laughs a little. I turned slowly and said to the guy sternly 'Sorry m8, do I know you?'. I've been waiting for the opportunity to say it for a while and I see the guy fairly frequently in the streets.
If "Sorry mate, do I know you?" is the best comeback you came up with after thinking over and over when you see this guy on the streets, you need to be more creative. Maybe some weed would help.