How does the author describe the Canterville ghost? What was the expression on the ghost's face?

The Canterville ghost appears in various forms.  The story even mentions that the ghost takes on the form of "Headless Earl" to frighten residents of the house.  That makes it difficult to describe a single form and look of the ghost.  It also makes it difficult to describe the expression on the ghost's face without knowing exactly which scene in the story that you are referring to.  I have a feeling that you are asking about the author describes the ghost's first appearance to the Otis family.

The ghost, Sir Simon, makes his first appearance at the beginning of section two, and he only appears for Mr. Otis.  The text says that it was exactly one in the morning.  The entire Otis family is asleep, but Mr. Otis is woken up by a strange noise out in the hall. Spooky.  

Some time after, Mr. Otis was awakened by a curious noise in the corridor, outside his room. It sounded like the clank of metal, and seemed to be coming nearer every moment.

  So, being the man of the house, Mr. Otis gets up to investigate the strange noises.  Mr. Otis opens his bedroom door and is immediately face to face with the ghost of Sir Simon.  Despite the fact that the story is a comedy, the description is terrifying.  

. . . an old man of terrible aspect. His eyes were as red burning coals; long grey hair fell over his shoulders in matted coils; his garments, which were of antique cut, were soiled and ragged, and from his wrists and ankles hung heavy manacles and rusty gyves.

That's scary.  Burning eyes of coal!  I'd run.  But not Mr. Otis.  He calmly tells Sir Simon to stop making so much noise and then proceeds to hand the ghost a bottle of oil to lube up his chains, so they don't make any more noise.  Then Mr. Otis turns around and goes back to bed.  

The text doesn't explicitly describe what Sir Simon's facial expression was, but the text does indicate that he just stood there in a dumbfounded stupor.  Just stood there!  I imagine his expression was a mixture of just complete blankness with a hint of incredulity.  Maybe his jaw hung open just a bit.  Then he ran down the hall in a sort of angry "pity party" for himself.  

For a moment the Canterville ghost stood quite motionless in natural indignation; then, dashing the bottle violently upon the polished floor, he fled down the corridor, uttering hollow groans, and emitting a ghastly green light.