moss-grown vs stick-in-the-mud

moss-grown is not a noun while stick-in-the-mud is a noun.

Word	Noun	Adjective	Verb	Adverb
moss-grown	No	Yes	No	No
stick-in-the-mud	Yes	Yes	No	No

As adjectives, moss-grown and stick-in-the-mud are synonyms defined as:

moss-grown and stick-in-the-mud: (used pejoratively) out of fashion; old fashioned

Other synonyms of moss-grown include fogyish, mossy, stodgy.

moss-grown (noun)	stick-in-the-mud (noun)
	someone who moves slowly

moss-grown (adjective)	stick-in-the-mud (adjective)
overgrown with moss	(used pejoratively) out of fashion; old fashioned
(used pejoratively) out of fashion; old fashioned