The Savior had previously instructed the Nephites concerning the proper language of prayer (see 3 Nephi 13:9).
They knew that they should pray “unto the Father, in my name” (see 3 Nephi 18:23), yet under the influence of the Spirit they prayed to Jesus “calling him their Lord and their God.”
He was and is both Redeemer and God. In reverential worship they directed their prayers to the Savior, and he did not stop them nor correct them. It appears that, in this case, it was appropriate because a resurrected God stood in their very presence (see verse 22).
Elder Bruce R. McConkie has written:
“Jesus was present before them as the symbol of the Father. Seeing him, it was as though they saw the Father; praying to him, it was as though they prayed to the Father. It was a special and unique situation that as far as we know has taken place only once on earth during all the long ages of the Lord’s hand-dealings with his children.”
(Promised Messiah, p. 561.)