Encore wa ika ga is the phrase the mermaids say at the end of a song (After "Love Shower Pitch!"). It means "Do you want an encore?" The most of the times after the phrase villains teleportate themselves somewhere else to go away from the mermaid princesses, because they don't want to hear more.
However, when Hanon, Rina, and Lucia sing in public (eg. Episode 38 ) they don't normally say that, the audience just shouts 'Encore! Encore! Encore!' and they sing another song to please the audience.