Matt doesn't tell David about his ghost. He'd only be disappointed.
She's a temporal echo, an image, a vision from Matt's future - Matt knows it's
/his/ future, he can feel it, the resonance from the echo, feels like it's
coming from /his/ timeline.
Pale. Red-haired. Striking. Matt thinks she looks interesting, wonders who she
is, what she'll be like.
He's tried asking, but she mimes zipping her lips closed, says nothing, letting him wonder.
The Doctor brothers have always looked to the future, eager to see what happens
next. Matt thinks he's going to like meeting his ghost for real.