Taking a shower, found someone peep from a window, how to do?

Recently I move in a flat to share with the other 2 girls. Yesterday morning, when I was taking a shower, I felt something. I unconsciously looked at a window underneath our flat, I saw someone was peeping from a window of his flat. I closed the window in the bathroom immediately. However, I began to worry. I am worried whether he could do something further. Even the flat I live in is a typical residential building with security guard.However, there is no CCTV camera in each floor. The cameras only are in lifts.

What should I do? I became worried and I didn't sleep well last night.

Could some one give some advice? I am thinking to move out, however I don't want to move frequently as I just move in.

Thanks in advance.

Close all the windows !!! The worse comes to the worse, call the police!

Close the window, put in plastic shower curtains...

don't blame if you shower with open windows...