When they bump into Frisk, (well, not really bump so much as the human's shoulder moves a little through their body) they are hit with two distinct memories of the child- one of the underground and one of Genosha. They stare at them for a moment. The music is too loud to talk over and they can't pretend that the fondness they felt as a human for Frisk was gone and their normal solution- to phase out of the situation as quickly as possible- is not an option at their cousin's birthday performance.

So Napstablook smiles a little, awkwardly, and start to sway to the rhythm, watching for the child's reaction.