What is the term for a common expression or colloquialism that is inaccurate or misleading, such as the use of "mental math" to mean "mental calculation" or "mental arithmetic"?
I think you are looking for the word metonymy: a figure of speech in which a thing or concept is called not by its own name but rather by the name of something associated in meaning with that thing or concept.
It is closely related to the more general term synecdoche.
Edit: Now I've read your comments I like the answer of user49727 (catachresis) and think that one is best suited to your demands (the negative connotation). You might also consider faulty generalization and to lump together.
This should come under the rhetorical device called catachresis. Etymologically catachresis (from Greek kata- implying perversion) embodies the sense of misuse (and hence of misleading) you require.
misuse or strained use of words, as in a mixed metaphor, occurring either in error or for rhetorical effect.
With "appropriate" meaning "to attribute to a cause or meaning", in this case.
To misappropriate a term means to use it in the wrong way/with the wrong meaning.
the use of an incorrect word in place of a word with a similar sound, resulting in a nonsensical, often humorous utterance.
An example is Yogi Berra's statement: Texas has a lot of electrical votes (rather than electoral votes).
The term derives from the French mal à propos meaning inappropriate and was used as the name of a character, Mrs. Malaprop, in an 18th century play.
An alternative term is a Dogberryism from Officer Dogberry in Shakespeare's Much Ado about Nothing.