Moon Sea Back Floor
The Moon Sea has a traditional back floor entry much like the first two dungeons and the Sun & Moon Temple. You will need to collect an item and then use it at the designated area on the map. After doing this you'll be given the option to "test your faith" and jump or wuss out. Jumping will get you to the back floor of the Moon Sea. The item you are seeking for back floor access in the Moon Sea is the Bravery Launch.
The only other thing I can even possibly think of mentioning about the back floor here is each character gets a unique cutscene when they enter the Moon Sea Back Floor. It may be worth entering and exiting on each character to see it.