My neighbour texts me but ignores me in real life

I have a neighbour since years back who lives in our apartment building and a few years ago he somehow was able to find me on Messenger and im not sure how.
He started texting me that we met outside so I asked him when and he told me it was when I walked past him once and I was like oookay...
Then hes has had convos with himself in my Messenger on and off ever since but I never reply to him. We frequently see each other outside or run into each other in the stairwell but he just looks at me and doesnt say anything or even nod at me or anything.
He had left me alone for a while but ive been seeing this guy this summer and almost everytime I was walking with him outside we'd see my neighbours parents outside and walked past them and then after that kept happening my neighbour started texting me again. I got a little curious so I responded back one evening because it was really suspicious because the guy ive been seeing had just texted me good night and literally 2 seconds later my neighbour texts me hello and this has kept happening lately which is almost creepy... I only responded to like 2 messages then I was like nope and stopped. Then the next day he went "I just want to be friends" so I was like "okay thats fine but right now im too busy to talk" because I felt sorry for him when he said that. I dont wanna make things hostile between us either you know because we're neighbours and all.
I last ran into him 2 days ago down in the stairwell and I figured since he's texted me so much lately he's gotta say something... So I actually made eye contact this time and smiled a little to be polite but what did he do? he just glanced at me with his always serious neutral face and walked past me. Then the next day he texted me "hi whats up?" which I ignored because I dont get why I should bother to even be friendly. He acts like a total weirdo and for all I know hes a stalker who figured my name out by staring at the windows in the dark at night until he found me and knew which floor I live on so he could walk up here and get my name from the door and then add me on Messenger, and if so he may always be staring at me in the dark.
I wouldnt know. His dad is shady enough, hes always pacing around alone in the dark talking in an ear piece, sometimes you'll walk past a bush in the evening on your way to the store and he's just standing there behind the bush being all sneaky with his ear piece. No wonder his son is a creep. Who knows what that family is up to.