Here the 1920 LDS edition added the subordinate conjunction that after the verb remember. Besides Mosiah 16:13, we have four examples of “and remember (that)”, one of which occurs without the that in the earliest text (marked below with an arrow):
More generally, in the original text, after the verb remember we get that (with perhaps an intervening phrase or clause) most of the time (43 times), but occasionally the that is missing after the verb remember (5 times in addition to Mosiah 16:13 and Alma 5:52, listed above):
Of course, it is possible in Mosiah 16:13 that the original text had a that which was lost during the early transmission of the text. But since there are a number of examples without the that, the critical text will maintain the earliest text for Mosiah 16:13. For discussion of other verbs for which that has been accidentally lost or added, see under that in volume 3.
Summary: Remove the intrusive that in Mosiah 16:13 so that the text reads according to the earliest textual sources; the that does improve the smoothness of the text, but based on other usage in the original text, the subordinate conjunction is not necessary.