Is the Degenerate skill name used as a noun or a verb?
Some minor spoilers ahead. 

I think it is a noun. Its other name is 'Deviant', which is used as a noun or an adjective. Furthermore, it was noted in the wiki that


  The kanji for Deviant translate to both Shapeshifter and Deviant (Degenerate or Pervert)


which


   is where the pun comes from when Rimuru teases the spirit of Shizue inside him at a comedic panel of the manga at the end of the chapter. 


From these two translations, it is also used as a noun.

Degenerate, according to Wikipedia, is someone who has fallen from a previous state whereas deviant is something that departs from the norm. The skill releases properties inherent to the target, which is quiet similar to what happens when one becomes a degenerate or if someone becomes a deviant.