falloff vs falling off

falloff and falling off both are nouns.

Word	Noun	Adjective	Verb	Adverb
falloff	Yes	No	No	No
falling off	Yes	No	No	No

As nouns, falloff and falling off are synonyms defined as:

falloff and falling off: a noticeable deterioration in performance or quality

Other synonyms of falloff include drop-off, slack, slump.

falloff (noun)	falling off (noun)
a noticeable deterioration in performance or quality	a noticeable deterioration in performance or quality