The Doctor's curiosity is met by a bit of apprehension and alarm at how casually he's being responded to. It was an errant thought...and having sensed no Mu nearby, the last thing Blue expects is to be responded to.

And if he were responded to? He'd imagine it would be out of revilement or fear.

"Invade," Blue echoes, only partly invested in the answer. It's more the person who speaks that's of interest to him now.

Curiosity now meets curiosity as the Mu turns to look at this stranger.

"You...heard my thoughts. Even though I did not send them to you with intent." And he isn't bothered by it!