at rest vs deceased

at rest is not a noun while deceased is a noun.

Word	Noun	Adjective	Verb	Adverb
at rest	No	Yes	No	No
deceased	Yes	Yes	No	No

As adjectives, at rest and deceased are synonyms defined as:

Other synonyms of at rest include asleep, at peace, departed, gone.

at rest (noun)	deceased (noun)
	someone who is no longer alive

at rest (adjective)	deceased (adjective)
in a state of repose or especially sleep	dead
dead