However, Graendal might be making an assumption she's posing as an Aes Sedai, figuring that Mesaana wouldn't pose as a servant, novice or accepted. (As far as we know, Mesaana hasn't confided her disguise to anyone.)

Also, her agents aren't "keeping an eye on Mesaana", they are informing Graendal about the plots she is undertaking, which is a big difference.

All-in-all, I think there is less solid evidence in this pov than what it first appeared. I, myself, was hoping for more, as I think she is posing as Danelle, too.