Shockingly, there are particular things on the minds of these teenage boys, and they get their science logic from Doctor Who. Granted, I don't recall the actual explanation for why Weeping Angels from the series can't move when being looked at, but I have no doubt that "quantum wibbly wobbly" is as satisfactory an answer as any.
And yes, they are comparing Grace to a Weeping Angel. Instead of being some angel that looks like an angel statue that does something something sends back in time something and can't move if being looked at, the theory about Grace is she's a magic lady that changes things about herself when you're not looking, and could potentially do so in a way you could super like.
So it's not exactly like teasing a Weeping Angel and hoping for a good outcome, but there are comparisons to be made.
And no, I don't know how we can see Duck's eyebrows over his hat in panel two. We all just gotta deal with it.