I don’t know the word (if their is one) for some one who gets expressions just slightly wrong, malaprops surely refer just to someone who gets a word wrong.
Samuel Goldwyn was famous for several (which the charitable put down to his lack of command of the English language) such as “Include me out ” and “You’ve got to take the bitter with the sour.”.

I remember the mother of a friend of ours who decided to adopt the language of her teenage children but had the ability to get it slightly wrong.
To express her lack of enthusiasm for a project she was apt to grunt “Great Deal” under the mistaken impression that she was saying “Big Deal”.

Just recently a friend who works in a radio station told me of a boss who, when afraid that she was going too far, used to tell her “Now don’t go upsetting the apple tart ”
That one took me several moments to work out.