in what way did the face at the window appear inhuman to Grant Munro

From the text:

I was some little way off, so that I could not make out the features, but there was something unnatural and inhuman about the face. That was the impression that I had, and I moved quickly forwards to get a nearer view of the person who was watching me. But as I did so the face suddenly disappeared, so suddenly that it seemed to have been plucked away into the darkness of the room. I stood for five minutes thinking the business over, and trying to analyze my impressions. I could not tell if the face were that of a man or a woman. It had been too far from me for that. But its color was what had impressed me most. It was of a livid chalky white, and with something set and rigid about it which was shockingly unnatural.