sandman vs gremlin

sandman and gremlin both are nouns.

Word	Noun	Adjective	Verb	Adverb
sandman	Yes	No	No	No
gremlin	Yes	No	No	No

As nouns, gremlin is a hypernym of sandman; that is, gremlin is a word with a broader meaning than sandman:

sandman: an elf in fairy stories who sprinkles sand in children's eyes to make them sleepy
gremlin: (folklore) fairies that are somewhat mischievous

Other hypernyms of sandman include brownie, elf, hob, imp, pixie, pixy.

sandman (noun)	gremlin (noun)
an elf in fairy stories who sprinkles sand in children's eyes to make them sleepy	(folklore) fairies that are somewhat mischievous