The plot of Moana is the exact opposite of The Little Mermaid:

The Little Mermaid is about an ocean princess who wants to be on land, while Moana is about a land princess who wants to be out on the ocean.

The villain in The Little Mermaid pretends to be a friend, while the villain in Moana turns out to actually be a friend.

In The Little Mermaid, a man sings a song about eating a crab (Sebastian), while in Moana, a crab (Tamatoa) sings a song about eating humans.