"Who Is This "Moon"?" is an instrumental score for Sing.
It can be heard when the bears confront Mike about the money he cheated them out of.
Mike tells the bears that Buster Moon has the amount of money they are looking for, but it is then revealed that Buster has lied about having the money, much to the shock of the contestants and the fury of the bears.
A large crack then forms in the glass floor and wall, and the music ends right before all the glass shatters and floods the Moon Theater.
The score's title refers to a quote from the bear leader, when he asks who Buster is.