Beautiful symbolism, really, if you know the deeper meaning. So the moral of the story is that Moana conquers her demons. This is an allegory for the Self. The demon is Moana.
Notice at the beginning she shows a shining green stone. This is symbolic of the heart. In many esoteric traditions green was associated with the heart, and if you pay attention, she later places the shining green stone in the demons heart, replacing the darkness with light. So yeah, I'm not stretching here.
Also, notice how she has to initiate the encounter. She couldn't have defeated her demon from across the sea, and she says she crossed the horizon to find her, as in she had to journey into the Self to find her demon. Again, beautiful symbolism.
After she slays her demon, and the demon's heart is restored, she symbolically becomes a goddess. These sort of stories have been told since time immemorial, and they're riddled throughout every culture.