slacken off vs decrease

slacken off is not a noun while decrease is a noun.

Word	Noun	Adjective	Verb	Adverb
slacken off	No	No	Yes	No
decrease	Yes	No	Yes	No

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

Other hypernyms of slacken off include diminish, fall, lessen.

slacken off (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

slacken off (verb)	decrease (verb)
become less intense	decrease in size, extent, or range
	make become smaller