fecal matter vs excrement

fecal matter and excrement both are nouns.

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

As nouns, excrement is a hypernym of fecal matter; that is, excrement is a word with a broader meaning than fecal matter:

fecal matter: solid excretory product evacuated from the bowels
excrement: waste matter (as urine or sweat but especially feces) discharged from the body

Other hypernyms of fecal matter include body waste, excreta, excretion, excretory product.

fecal matter (noun)	excrement (noun)
solid excretory product evacuated from the bowels	waste matter (as urine or sweat but especially feces) discharged from the body