to conceal or cover part of something from view. The trees masked the city dump out, so it could not be seen from the street.The trees masked out the dump.
if someone's mask slips, they do something which shows people their real personality, when they have been pretending to be a different, usually nicer, type of person
Usage notes: A mask is something that covers your face to hide it.
His mask had suddenly slipped, and she saw him as the angry and cruel man that he really was.