When a guy cums on a girls face, it is liquid and wet. The the cum dries if it isn't wiped off, drying to the face causing a hard, dryness to form, and is white, looking like a mask.
Best used in a bukkake situation where lots of cum is involved. The girl would end up with so much cum on her face that it would cover it entirely. If it were to try, it would be similar to a mask.