Joseph Smith taught that, “the doctrine of translation is a power which belongs to [the] Priesthood.” (The Teachings of the Prophet Joseph Smith, p. 170) Those known to have been translated include Moses, Elijah, John the Beloved, the three Nephites, the entire city of Enoch, Alma, and presumably Nephi (1 Nephi 1:3). “Their place of habitation is that of a terrestrial order, and a place prepared for such characters He held in reserve to be ministering angels unto many planets, and who as yet have not entered into so great a fullness as those who are resurrected from the dead.” (Teachings, p. 170)
We know of something of the ministry of all of the above individuals except Alma and Nephi. Moses and Elijah revealed the sealing power to Peter, James, and John. John the Revelator and the three Nephites were preserved to bring souls unto Christ (3 Nephi 28:6,9). Enoch was to minister, “unto terrestrial bodies, of whom there has been but little revealed.” (Teachings, p. 170)
But we know nothing of the terrestrial ministry of Alma. Truly, ‘he was never heard of more’ (v. 18), and we are left to wonder about a 2000-year-old rumor which Mormon neither discounts nor verifies. Of the nature of his continuing ministry we can only speculate.
Joseph Fielding Smith
"…we understand why Elijah and Moses were preserved from death: because they had a mission to perform, and it had to be performed before the crucifixion of the Son of God, and it could not be done in the spirit. They had to have tangible bodies. Christ is the firstfruits of the resurrection; therefore if any former prophets had a work to perform preparatory to the mission of the Son of God, or to the dispensation of the meridian of times, it was essential that they be preserved to fulfill that mission in the flesh. For that reason Moses disappeared from among the people and was taken up into the mountain, and the people thought he was buried by the Lord. The Lord preserved him, so that he could come at the proper time and restore his keys, on the heads of Peter, James, and John, who stood at the head of the dispensation of the meridian of time. He reserved Elijah from death that he might also come and bestow his keys upon the heads of Peter, James, and John and prepare them for their ministry.
“But, one says, the Lord could have waited until after his resurrection, and then they could have done it. It is quite evident, due to the fact that it did so occur, that it had to be done before; and there was a reason. There may have been other reasons, but that is one reason why Moses and Elijah did not suffer death in the flesh, like other men do.” (Doctrines of Salvation, vol. 2, pp. 110-111.)