grey-headed vs hoar

grey-headed is not a noun while hoar is a noun.

Word	Noun	Adjective	Verb	Adverb
grey-headed	No	Yes	No	No
hoar	Yes	Yes	No	No

As adjectives, grey-headed and hoar are synonyms defined as:

grey-headed and hoar: showing characteristics of age, especially having grey or white hair

Other synonyms of grey-headed include gray-haired, gray-headed, gray, grey-haired, grey, grizzly, hoary, white-haired.

grey-headed (noun)	hoar (noun)
	ice crystals forming a white deposit (especially on objects outside)

grey-headed (adjective)	hoar (adjective)
showing characteristics of age, especially having grey or white hair	showing characteristics of age, especially having grey or white hair