Ancient masks were used in dramatic performances. Fitted with wigs, they covered the actor's whole head or were face masks. Made of linen, bark, leather, or clay, they were shaped with exaggerated human features and painted. Masks enabled the audience to identify a wide variety of stock characters from a distance, before they were introduced or spoke, thus serving as a kind of visual playbill.
Here Umbricius describes a whitened (pallentis) mask that is terrifying to the very young, either because of its color or because of the figure it represents -- villain, ghost or monster ( in Epigrammata 14.176, Martial describes a persona Germana, a "mask of a red-headed Batavian," as laughable to adults but fearsome to a child).