A Hockey Mask is a sex term that is used to describe cum that has dried on a girls face. Usually large amounts of cum are required.
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.
"I came all over that sleeping girls face last night. It looked like she had a hockey mask on her face when she woke up"