I recently took another look at Season 3 Episode 6: The Last Splash. There I noticed two details.
Detail 1:
When Bean mentions that she freed Elfo from hell, the mention of hell doesn't seem to impress Mora. As if this place was a completely normal one. It almost seems like Mora knows this place, or maybe has been there before.
Detail 2:
When Mora is welcomed by her family on Mermaid Island, Bean reveals that she is human, the mermaids recoil. As if they were afraid of humans.
It makes me wonder why the relationship between human and mermaid is the way it is. Both are afraid of each other and actually without reason. The mermaids are said to attract men to their death. Which is not true. And people think that mermaids are monsters, although they clearly show signs of goodness and intelligence.
Could it possibly be the myths that remained after the great battle? Or do the mermaids have a connection to hell and thus to Maru.