faecal matter vs excrement

faecal matter and excrement both are nouns.

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

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

faecal 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 faecal matter include body waste, excreta, excretion, excretory product.

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