dead-men's-fingers vs dead-man's-fingers

dead-men's-fingers and dead-man's-fingers both are nouns.

Word	Noun	Adjective	Verb	Adverb
dead-men's-fingers	Yes	No	No	No
dead-man's-fingers	Yes	No	No	No

As nouns, dead-men's-fingers and dead-man's-fingers are synonyms defined as:

dead-men's-fingers and dead-man's-fingers: the fruiting bodies of the fungi of the genus Xylaria

Other synonyms of dead-men's-fingers include Xylaria polymorpha.

dead-men's-fingers (noun)	dead-man's-fingers (noun)
the fruiting bodies of the fungi of the genus Xylaria	the fruiting bodies of the fungi of the genus Xylaria