mull over vs question

mull over is not a noun while question is a noun.

Word	Noun	Adjective	Verb	Adverb
mull over	No	No	Yes	No
question	Yes	No	Yes	No

As verbs, question is a hyponym of mull over; that is, question is a word with a more specific, narrower meaning than mull over:

Other hyponyms of mull over include premeditate, theologise, theologize, introspect, bethink, cogitate, wonder, puzzle, consider, study.

mull over (noun)	question (noun)
	uncertainty about the truth or factuality or existence of something
	an instance of questioning
	the subject matter at issue
	a formal proposal for action made to a deliberative assembly for discussion and vote
	a sentence of inquiry that asks for a reply
	an informal reference to a marriage proposal

mull over (verb)	question (verb)
reflect deeply on a subject	pose a question
	challenge the accuracy, probity, or propriety of
	place in doubt or express doubtful speculation
	pose a series of questions to
	conduct an interview in television, newspaper, and radio reporting