ebb vs fall back

ebb is a noun but fall back is not a noun.

Word	Noun	Adjective	Verb	Adverb
ebb	Yes	No	Yes	No
fall back	No	No	Yes	No

As verbs, fall back is a hypernym of ebb; that is, fall back is a word with a broader meaning than ebb:

ebb (noun)	fall back (noun)
the outward flow of the tide
a gradual decline (in size or strength or power or number)

ebb (verb)	fall back (verb)
fall away or decline	go back to bad behavior
hem in fish with stakes and nets so as to prevent them from going back into the sea with the ebb	retreat
flow back or recede	hang (back) or fall (behind) in movement, progress, development, etc.
	fall backwards and down
	move back and away from
	have recourse to