The phrase "motionless face" is correct and usable in written English.
It can be used to describe a person's face which shows no emotion. For example, "He stared at me with a motionless face, and I could not tell what he was thinking."
She was floating, motionless, face down in the water.
Wilson was motionless, face down, as the field rounded the first turn.
We moved on, and suddenly, in the middle of the wet, glistening walk, we saw a man lying motionless, face down.
Falling to her knees, she embraced the inert body and frantically tried to bring the young woman back to life - shaking her lifeless shoulders, stroking her hair and shouting a question into her motionless face.
Ed has very little to say to the big talkers who surround him, and Mr. Thornton's voice-over narration, playing over shots of his motionless face, emphasizes the character's essential alienation.
He used this great, sad, motionless face to suggest various related things: a one-track mind near the track's end of pure insanity; mulish imperturbability under the wildest of circumstances; how dead a human being can get and still be alive; an awe-inspiring sort of patience and power to endure, proper to granite but uncanny in flesh and blood.
There was the default glassy, motionless look – the kind of face you might pull after being chased into a wax museum by gangsters and trying to blend in with the exhibits.
On the back a stranger is caught by the camera paralysed in mid-sprint, while Drake leans against a wall quite motionless, his face betraying no semblance of an expression.
Scott is motionless, his face down in the ground.
He came to me when the massive stroke rendered his right leg and arm nearly motionless, his face crooked and his speech slurred.
"He was lying motionless and face down on the trampoline and two young boys who work there lifted him and moved him off the trampoline.
