decline vs drop off

decline is a noun but drop off is not a noun.

Word	Noun	Adjective	Verb	Adverb
decline	Yes	No	Yes	No
drop off	No	No	Yes	No

As verbs, drop off is a hyponym of decline; that is, drop off is a word with a more specific, narrower meaning than decline:

decline (noun)	drop off (noun)
a downward slope or bend
a gradual decrease; as of stored charge or current
change toward something smaller or lower
a condition inferior to an earlier condition; a gradual falling off from a better state; decline

decline (verb)	drop off (verb)
grow smaller	change from a waking to a sleeping state
grow worse	fall to a lower standard
show unwillingness towards	fall or diminish
inflect for number, gender, case, etc.	retreat
go down	remove (cargo, people, etc.) from and leave
fall in value
not accept as true