Wikipedia mentions mistake as a linguistic term in the context of language learners, found at Error (linguistics) (via Malapropism) citing H. Douglas Brown (1994) for a definition
a failure to utilize a known system correctly
I see no reason not to apply that to L1 speakers, unless there's a specific term to note the distinction. Linguistics has technical jargon and thus is a language variety, and even L1 speakers learn new words.
Your example using false in stead of fake shows half-way similar sound and meaning, after all, leaves little doubt that you were trying for fake news, but with a twist. The precise nature of the twist cannot be infered from the anecdote alone. If it were completely intentional you probably would not question it. Nevertheless you might have been aware of the mismatch, going with it anyway for several reasons, ideally improving the idiom, or at a minimum producing a coherent sentence.
Extreme cases of language confusion appear in Aphasia, where speech production can appear dysfunctional in various ways, due to brain damage, temporary or permanent. The respective research probably have jargon for the different varieties of mistakes.