For an unknown reason, Mosiah declines to judge and returns the problem to Alma. Probably he saw the problem as a religious one which must be handled by the proper religious authority. It seems likely that, understanding the development of diversity within the community, Mosiah understands that he must rule over a kingdom of diverse beliefs without alienating part of the community. For this reason, perhaps, he observes the religious/social distinction by declining to judge.
We know that Mosiah is a believer and therefore in sympathy with Alma’s teachings. His action separates religious functions from direct ties to government. By giving the decision to Alma, Mosiah can separate himself from that decision and therefore from its possible social repercussions. Mosiah is thus retaining his ability to govern the entire body, which includes an increasing number of religious dissenters and their sympathizers.