droop vs drop down

droop is a noun but drop down is not a noun.

Word	Noun	Adjective	Verb	Adverb
droop	Yes	No	Yes	No
drop down	No	No	Yes	No

As verbs, drop down is a hypernym of droop; that is, drop down is a word with a broader meaning than droop:

droop: droop, sink, or settle from or as if from pressure or loss of tautness
drop down: fall or descend to a lower place or level

Other hypernyms of droop include drop, sink.

droop (noun)	drop down (noun)
a shape that sags

droop (verb)	drop down (verb)
become limp	fall or descend to a lower place or level
droop, sink, or settle from or as if from pressure or loss of tautness
hang loosely or laxly