fell vs slip away

fell is a noun but slip away is not a noun.

Word	Noun	Adjective	Verb	Adverb
fell	Yes	Yes	Yes	No
slip away	No	No	Yes	No

As verbs, slip away is a hypernym of fell; that is, slip away is a word with a broader meaning than fell:

Other hypernyms of fell include elapse, glide by, go along, go by, lapse, pass, slide by, slip by.

fell (noun)	slip away (noun)
the act of felling something (as a tree)
seam made by turning under or folding together and stitching the seamed materials to avoid rough edges
the dressed skin of an animal (especially a large animal)

fell (adjective)	slip away (adjective)
(of persons or their actions) able or disposed to inflict pain or suffering

fell (verb)	slip away (verb)
cause to fall by or as if by delivering a blow	for time to move forward
sew a seam by folding the edges	leave furtively and stealthily
pass away rapidly