Warning: spoilers ahead
Quite simple, my theory is that Marlo is a descendant of Eris from A Mermaid Tale. If we consider this timeline, and say that Malibu is a descendant of Merliah, this would explain why Marlo's crown gave powers to Malibu: because she was born with powers and not Marlo, just like Merliah and Eris.
Thoughts?