decay vs moulder

decay is a noun but moulder is not a noun.

Word	Noun	Adjective	Verb	Adverb
decay	Yes	No	Yes	No
moulder	No	No	Yes	No

As verbs, moulder is a hyponym of decay; that is, moulder is a word with a more specific, narrower meaning than decay:

Other hyponyms of decay include decompose, molder, rot, go bad, spoil, putrefy, deliquesce.

decay (noun)	moulder (noun)
the organic phenomenon of rotting
a gradual decrease; as of stored charge or current
the process of gradually becoming inferior
the spontaneous disintegration of a radioactive substance along with the emission of ionizing radiation
an inferior state resulting from the process of decaying

decay (verb)	moulder (verb)
fall into decay or ruin	break down
lose a stored charge, magnetic flux, or current
undergo decay or decomposition