face mask vs ski mask

face mask and ski mask both are nouns.

Word	Noun	Adjective	Verb	Adverb
face mask	Yes	No	No	No
ski mask	Yes	No	No	No

As nouns, ski mask is a hyponym of face mask; that is, ski mask is a word with a more specific, narrower meaning than face mask:

face mask: mask that provides a protective covering for the face in such sports as baseball or football or hockey
ski mask: a woolen face mask to protect the face from cold while skiing on snow

Other hyponyms of face mask include catcher's mask, face guard, fencer's mask, fencing mask.

face mask (noun)	ski mask (noun)
mask that provides a protective covering for the face in such sports as baseball or football or hockey	a woolen face mask to protect the face from cold while skiing on snow