Umi no Triton

My mom plays Pipi in this. The series was made before I was born (1973). She also sang the ending credits theme song I think.

Green-haired orphan Toriton and his sassy mermaid friend Pipi are the 2 surviving descendants of Atlantis, the mythical Greek island that sank during an earthquake.

Toriton washes up on the shore and is raised by a kindly fisherman. He has superpowers underwater, but kids tease him about his green hair and what not. One day he discovers a glowing red dagger stashed away in the closet. Also a conch shell that contains a pre-recorded message from him parents, explaining that they had to send him off to sea before Atlantis sank.

They team up with a wise, matronly dolphin, meet a Yoda-like giant turtle who dies, and then go fight Poseidon (=Neptune in Roman mythology) and his evil minions with his red dagger.

Very much like Star Wars and Superman, plus Greek mythology stuff thrown in