I'm of 2 minds on this.
1) he could just be a little off and not have boundaries, yet perfectly harmless.
2) he could be really scary and your instincts are dead-on.
When in doubt, go with 2.
I once had a neighbor who was a little off but he gave me Manson vibes. He was a little guy and I had just moved over 1000 miles to avoid a stalker, having taken a year's worth of karate lessons. I figured I could probably take him in a fight if I had to, and I had a huge boyfriend I asked to come over and just let him get a look at to discourage him from talking to me. He scoffed at me and said I was overreacting.
This guy would pop up as I was coming out of my apartment door, out of the laundry room, getting out of my car coming home, show up at the grocery store across the street from the apartment complex...you get the idea. One day I come home from work and his face is on the front page of my newspaper, he's killed one of my neighbors who went inside his apartment for a beer, something he was always asking me to do.
Overreacting my @$$
Trust your instincts. Always.
Make a report with the police just in case, and communicate with a trusted neighbor, maybe make arrangements to keep an eye out for each other. I don't think there is such a thing as being too careful.