I have a neighbor who stares at me?

I have been living in my apartment for the past 4 years and in that time the apartment across the way from me has had two changes of tenants. Before there were 4 girls living there and all was fine. They moved out late last year and the new tenants moved in. From what it looks like its 2 guys and one girl living there now.

I have no problem with the girl or guy, but the other guy who is the younger of the men, keeps staring at me.

He sits in the apartment with the balcony door wide open and sits facing this way. I did a test the other day to see what would happen. I was outside sitting on my balcony floor ( he can't see me when I am sitting on the balcony floor) but I can see the reflection of this apartment on my door. Each time I moved the door, he looked this way.

He even stands in such a position to look through my slightly ajar door. He moves way over to the left of his balcony when there is plenty of space to stand elsewhere. His staring creeps me out. He sits in the apartment with the balcony door wide open and sits facing this way, which he is doing now. He is the only one with a door opened as it is quite chilly out at the moment.

I feel quite uncomfortable knowing someone is sitting in the shadow of their room staring over at me. It's stressing me.

This is genuine, so please genuine helpful replies only.

Thanks