slack up vs decrease

slack up is not a noun while decrease is a noun.

Word	Noun	Adjective	Verb	Adverb
slack up	No	No	Yes	No
decrease	Yes	No	Yes	No

As verbs, decrease is a hypernym of slack up; that is, decrease is a word with a broader meaning than slack up:

Other hypernyms of slack up include lessen, minify.

slack up (noun)	decrease (noun)
	the act of decreasing or reducing something
	the amount by which something decreases
	a change downward
	a process of becoming smaller or shorter

slack up (verb)	decrease (verb)
make less active or fast	decrease in size, extent, or range
	make become smaller