I believe in most states wearing a mask to covers one face, if not for costume partys, itself is a felony to conceal of the identity of the person. So that of itself is a crime.
States(and some City's) vary widely as to the ruling of this law including whether it's a felony or misdemeanor. Too, in the states/city's where it is a felony, it's usually a felony only in restricted areas and locations. A misdemeanor in unrestricted areas.