"Yes, that was me," Assuming that Benji is talking about the singing, since Clio can't think what else it would be, "Sorry if it was rude of me, it's just... been a while. And I liked your music."

She doesn't think there are rules against singing along - unless she'd been trying to steal paying costumers ("paying customers") from Benji - but the apology is there anyway, just in case. She doesn't want to make enemies here.