fecal matter vs melaena

fecal matter and melaena both are nouns.

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

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

fecal matter: solid excretory product evacuated from the bowels
melaena: 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, melena.

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