What can I do about a hostile work environment.

"Hostile work environment" is a technical legal phrase that applies ONLY in the context of employment discrimination. Your coworker is creating a hostile work environment for you if, and only if, he is stalking you for some discriminatory reason. You haven't mentioned anything about the reason why he is stalking you, but my experience has been that this kind of behavior usually happens as part of an ongoing pattern of sexual harassment. Is my guess correct? If I'm right, and your coworker is stalking you and creeping you out on the basis of your sex, you should report his conduct to either your supervisor or to someone in your company's HR department. Make sure you use words like "sexual harassment," so that they'll be on notice that you have a potential discrimination claim. You should also begin to keep a diary - log everything creepy that he does, as soon as you can write it down. Keep Sergeant Friday's injunction of, "Just the facts, Ma'am," in mind as you do it. Think like a journalist. Who did it? What did he do? What did he say? When? Where? Why? How did it make you feel? How did you react? Were there any witnesses? If I'm wrong and your coworker is an equal-opportunity creep, you should still report his conduct to your supervisor. Conduct like this is bad for employee morale even when it's not illegal. It might also violate your employer's internal rules.

This is very strange situation. How are you aware of this unusual behavior? Have you reported it to your supervisor? Even if this is not sexually motivated behavior as reasonably suggested it may be by one of my colleagues, it does not seem like behavior that a responsible employer would want in the workplace.