muster up vs muster

muster up is not a noun while muster is a noun.

Word	Noun	Adjective	Verb	Adverb
muster up	No	No	Yes	No
muster	Yes	No	Yes	No

As verbs, muster up and muster are synonyms defined as:

Other synonyms of muster up include come up, rally, summon.

muster up (noun)	muster (noun)
	compulsory military service
	a gathering of military personnel for duty

muster up (verb)	muster (verb)
gather or bring together	call to duty, military service, jury duty, etc.
	gather or bring together