falling off vs drop-off

falling off and drop-off both are nouns.

Word	Noun	Adjective	Verb	Adverb
falling off	Yes	No	No	No
drop-off	Yes	No	No	No

As nouns, falling off and drop-off are synonyms defined as:

falling off and drop-off: a noticeable deterioration in performance or quality

Other synonyms of falling off include falloff, slack, slump.

falling off (noun)	drop-off (noun)
a noticeable deterioration in performance or quality	a change downward
	a steep high face of rock
	a noticeable deterioration in performance or quality