And the trap is set

I think, like that Shiki guy, Flandre may have freaked out becuz she couldn't see Mokou's "eye"

No, Flandre was in the middle of saying something about humans prior.

Also, is Mokou trolling Keine~?

trying to cow her into submission?

Cutest Mokou's expression I've ever seen.

Aozora seems to carry this theme throughout his doujins. Flan is terrified of those of the Hourai because they can't die which Eirin says must be like seeing a skeleton walking around.