gray mullet vs percoid fish

gray mullet and percoid fish both are nouns.

Word	Noun	Adjective	Verb	Adverb
gray mullet	Yes	No	No	No
percoid fish	Yes	No	No	No

As nouns, percoid fish is a hypernym of gray mullet; that is, percoid fish is a word with a broader meaning than gray mullet:

gray mullet: freshwater or coastal food fishes a spindle-shaped body; found worldwide
percoid fish: any of numerous spiny-finned fishes of the order Perciformes

Other hypernyms of gray mullet include percoid, percoidean.

gray mullet (noun)	percoid fish (noun)
freshwater or coastal food fishes a spindle-shaped body; found worldwide	any of numerous spiny-finned fishes of the order Perciformes
highly valued lean flesh of marine or freshwater mullet