fall away vs lapse

fall away is not a noun while lapse is a noun.

Word	Noun	Adjective	Verb	Adverb
fall away	No	No	Yes	No
lapse	Yes	No	Yes	No

As verbs, lapse is a hyponym of fall away; that is, lapse is a word with a more specific, narrower meaning than fall away:

Other hyponyms of fall away include backslide.

fall away (noun)	lapse (noun)
	a mistake resulting from inattention
	a failure to maintain a higher state
	a break or intermission in the occurrence of something

fall away (verb)	lapse (verb)
fall to a lower standard	go back to bad behavior
diminish in size or intensity	drop to a lower level, as in one's morals or standards
	end, at least for a long time
	for time to move forward
	pass into a specified state or condition; sink into
	let slip