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

From the text:

“I don’t know what there was about that face,
Mr. Holmes, but it seemed to send a chill right
down my back. 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 busi
ness 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.