You can see Moses as a motif in this novel because of the Biblical significance of Moses and because of his significance to slave society in general.
In the Bible, Moses led the Israelites out of slavery in Egypt and to the Promised Land. Because of this, Moses was as much of a hero to slaves as Jesus was.
In the book, Huck can be said to be acting like Moses. You can argue that he is leading Jim out of slavery and to the promised land.
In a novel about someone being led out of slavery, Moses is a logical motif.