face, facial, full-face
gas
breathing, oxygen
dust
protective
surgical
hockey (AmE), ski
Halloween

have on, wear
don, put on
pull off, remove, strip off, take off, tear off
In the second part of the play, the actors take off their masks.

conceal sth, cover sth, hide sth
The man's face was hidden by a mask.

behind a/the mask, beneath a/the mask
Two eyes glared at him from beneath the mask.

in a/the mask
two men in black masks

mask over
She wore a mask over her face.

blank, cold, emotionless, expressionless, impassive, stony
Her face was a blank mask as she answered my question.

slip
For a moment her mask slipped, and I saw how scared she really was.

behind a/the mask
Behind the mask of friendliness, I know he really dislikes me.

mask for
His fooling around is a mask for his lack of confidence.

a mask of indifference
He was hiding behind a mask of indifference but she wasn't fooled.