wild crab vs crab apple

wild crab and crab apple both are nouns.

Word	Noun	Adjective	Verb	Adverb
wild crab	Yes	No	No	No
crab apple	Yes	No	No	No

As nouns, crab apple is a hypernym of wild crab; that is, crab apple is a word with a broader meaning than wild crab:

wild crab: wild crab apple native to Europe; a chief ancestor of cultivated apples
crab apple: any of numerous wild apple trees usually with small acidic fruit

Other hypernyms of wild crab include crabapple, wild apple.

wild crab (noun)	crab apple (noun)
wild crab apple native to Europe; a chief ancestor of cultivated apples	small sour apple; suitable for preserving
	any of numerous wild apple trees usually with small acidic fruit
	any of numerous varieties of crab apples cultivated for their small acidic (usually bright red) fruit used for preserves or as ornamentals for their blossoms