Mab is pretty sure Eliza is aware she is dead. The reaction tells her that she also knows what Faust's plans are and doesn't approve. Of course, she doesn't need to approve if he can summon her and force her into a body. With Clara nearby she's certainly not going to mention that aloud. This is Faust's show, he gets to decide what is explained to his friends and what isn't.

She touched Aradia's wrist just momentarily in a soothing gesture and murmured, "I believe she is aware of the implications."