alley cat vs Felis catus

alley cat and Felis catus both are nouns.

Word	Noun	Adjective	Verb	Adverb
alley cat	Yes	No	No	No
Felis catus	Yes	No	No	No

As nouns, Felis catus is a hypernym of alley cat; that is, Felis catus is a word with a broader meaning than alley cat:

Other hypernyms of alley cat include domestic cat, Felis domesticus, house cat.

alley cat (noun)	Felis catus (noun)
a homeless cat	any domesticated member of the genus Felis