The dreamscape itself doesn't change, not yet, but Merlin's definitely beginning to assert a certain... control over it. Slowly and carefully to avoid detection as long as possible. In his element though he may be, a mind that's aware--lucid--with consciousness near the surface, can still resist his efforts. Especially when the target is particularly powerful.

"Ah, you know, now that you mention it..." His tone is surprised, as though he'd only just realized, but the mischievous look in his eyes belies it. "Though unfortunately, it may be a little late to notice!"

And with that, Merlin makes his move. Not a physical one in any way, but a mental 'attack', so to speak, intending to wrap his target in a new dream of his devising. Assuming he'd been subtle enough to catch her off guard and unable to put up much of a fight, that is.