face mask vs face guard

face mask and face guard both are nouns.

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

As nouns, face guard is a hyponym of face mask; that is, face guard 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
face guard: face mask consisting of a strong wire mesh on the front of football helmets

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

face mask (noun)	face guard (noun)
mask that provides a protective covering for the face in such sports as baseball or football or hockey	face mask consisting of a strong wire mesh on the front of football helmets