Which is a direct violation of the constitution. Only congress the authority to control spending and Musk has absolutely zero authority to stop any payments congress has authorized. It is a naked power grab and musk should spend the rest of his life in prison for it.
You changed your wording to fit your argument. "To spend" became "to control spending", implicitly acknowledging that the two phrases have different meanings.
The executive branch does not have the authority to cut off congressionally appropriated spending. Congress specifically passed a law (Impoundment Control Act) to make that as explicit as could be
But they're not spending; they're kind of doing the opposite of spending. And reducing waste is a previously known (for a long time) goal of theirs.