grinning vs facial expression

grinning and facial expression both are nouns.

Word	Noun	Adjective	Verb	Adverb
grinning	Yes	No	No	No
facial expression	Yes	No	No	No

As nouns, facial expression is a hypernym of grinning; that is, facial expression is a word with a broader meaning than grinning:

grinning: a facial expression characterized by turning up the corners of the mouth; usually shows pleasure or amusement
facial expression: a gesture executed with the facial muscles

Other hypernyms of grinning include facial gesture.

grinning (noun)	facial expression (noun)
a facial expression characterized by turning up the corners of the mouth; usually shows pleasure or amusement	the feelings expressed on a person's face
	a gesture executed with the facial muscles