Mermaid Melody Pichi Pichi Pitch Pure

Synopsis

A man goes around looking for people to be idols. He asks Alala (who is in human form and accepts) and Rina. Rina (who is with Masahiro at the time) looks at the poster, before Masahiro grabs it out of her hands and rejects. When Masahiro sees Rina staring at a poster of Alala, who became a new idol, he thinks he ruined her dream of becoming an idol. The three mermaids end up at the concert and defeat Alala.

(Source: Wikipedia)