human waste vs excrement

human waste and excrement both are nouns.

Word	Noun	Adjective	Verb	Adverb
human waste	Yes	No	No	No
excrement	Yes	No	No	No

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

human waste: the body wastes of human beings
excrement: waste matter (as urine or sweat but especially feces) discharged from the body

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

human waste (noun)	excrement (noun)
the body wastes of human beings	waste matter (as urine or sweat but especially feces) discharged from the body