moorhen vs water hen

moorhen and water hen both are nouns.

Word	Noun	Adjective	Verb	Adverb
moorhen	Yes	No	No	No
water hen	Yes	No	No	No

As nouns, water hen is a hypernym of moorhen; that is, water hen is a word with a broader meaning than moorhen:

moorhen: black gallinule that inhabits ponds and lakes
water hen: any of various small aquatic birds of the genus Gallinula distinguished from rails by a frontal shield and a resemblance to domestic hens

Other hypernyms of moorhen include gallinule, marsh hen, swamphen.

moorhen (noun)	water hen (noun)
black gallinule that inhabits ponds and lakes	any of various small aquatic birds of the genus Gallinula distinguished from rails by a frontal shield and a resemblance to domestic hens
female red grouse	a coot found in North America