Is this harassment? What can I do?

Honestly, he isn't doing anything wrong. However, you should report this behavior to you manager. He or she should then ask him to stay off the premises. Then if he does return, it is indeed trespassing. If your manager doesn't do anything about it and the behavior persists, you can file a police report, which will result in the same thing. I wouldn't say that you are overreacting, you are just concerned for your safety, as you should be. Besides, that IS creepy.