faecal matter vs melena

faecal matter and melena both are nouns.

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

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

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

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

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