When Moroni visited Joseph Smith on the night of September 21, 1823, he did more than just tell him about “a book deposited, written upon gold plates,” though that was the primary purpose of his visit (Joseph Smith—History 1:34). He also quoted various biblical prophecies, indicating that they were soon to be fulfilled (JS—H 1:36–40). Among those prophecies was Malachi 3–4, but Joseph remembered that when quoting Malachi 4, there was “a little variation from the way it reads in our Bibles” (JS—H 1:36).
https://knowhy.bookofmormoncentral.org/knowhy/why-did-moroni-quote-malachi-about-elijahs-coming