Encore wa ika ga is the mermaids say at the end of a song (After "Love Shower Pitch!"). It means "Do you want an encore?" Most of the time, however, the enemines don't want to hear more, and respond by teleporting away from the mermaid princesses.
However, when Hanon, Rina, and Lucia sing in public (Episode 38) they don't normally say the phrase; the audience just shouts 'Encore! Encore! Encore!' and they sing another song to please them.