Mugil cephalus vs grey mullet

Mugil cephalus and grey mullet both are nouns.

Word	Noun	Adjective	Verb	Adverb
Mugil cephalus	Yes	No	No	No
grey mullet	Yes	No	No	No

As nouns, grey mullet is a hypernym of Mugil cephalus; that is, grey mullet is a word with a broader meaning than Mugil cephalus:

Mugil cephalus: most important commercial mullet in eastern United States
grey mullet: freshwater or coastal food fishes a spindle-shaped body; found worldwide

Other hypernyms of Mugil cephalus include gray mullet, mullet.

Mugil cephalus (noun)	grey mullet (noun)
most important commercial mullet in eastern United States	freshwater or coastal food fishes a spindle-shaped body; found worldwide
	highly valued lean flesh of marine or freshwater mullet