human face vs mug

human face and mug both are nouns.

Word	Noun	Adjective	Verb	Adverb
human face	Yes	No	No	No
mug	Yes	No	Yes	No

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

human face: the front of the human head from the forehead to the chin and ear to ear
mug: the human face (`kisser' and `smiler' and `mug' are informal terms for `face' and `phiz' is British)

Other hyponyms of human face include countenance, kisser, phiz, physiognomy, smiler, visage.

human face (noun)	mug (noun)
the front of the human head from the forehead to the chin and ear to ear	with handle and usually cylindrical
	the human face (`kisser' and `smiler' and `mug' are informal terms for `face' and `phiz' is British)
	a person who is gullible and easy to take advantage of
	the quantity that can be held in a mug

human face (verb)	mug (verb)
	To stare awkwardly
	rob at gunpoint or with the threat of violence