Graendal may know that Mesaana is in the white tower and have her own tower darkfriends/black ajah apprising her of what orders are coming from the top (Mesaana), and thus they could be keeping Graendal aware of what Mesaana is doing in the tower.

Nobody has yet satisfactorily explained how Mesaana would be able to impersonate a known aes sedai for an extended period. Also, I have not heard a satisfactory explanation as to how Mesaana would disguise her incredible strength in the power. Either she disguises her ability to channel entirely at all times (which would be extremely suspicious) or she has a heretofore unkown ability to make her strength in the power seem lower than it really is. (Someone else pointed out that there is such an ability, but gave no source for the assertion).

She may be posing as an Aes Sedai, but I still think it more likely she is posing as a Non-Channeler.