fecal matter vs melena

fecal matter and melena both are nouns.

Word	Noun	Adjective	Verb	Adverb
fecal matter	Yes	No	No	No
melena	Yes	No	No	No

As nouns, melena is a hyponym of fecal matter; that is, melena is a word with a more specific, narrower meaning than fecal matter:

fecal matter: solid excretory product evacuated from the bowels
melena: abnormally dark tarry feces containing blood (usually from gastrointestinal bleeding)

Other hyponyms of fecal matter include dog do, dog shit, dog turd, doggy do, crap, dirt, poop, shit, shite, turd, droppings, dung, muck, meconium, melaena.

fecal matter (noun)	melena (noun)
solid excretory product evacuated from the bowels	abnormally dark tarry feces containing blood (usually from gastrointestinal bleeding)