Okay Mei, I'll explain.
Tobi has been referred to as Mizukage by Fishface, so we know at one point he was the Mizukage, but fishface could simply have known that someone was controlling him, as we know the 4th Mizukage was controlled.
Now we see Danzo has Shisui's eye/whatever else, and as stated by the Kakashi wannabe, he had the power to control people, and also was awesome at Shunshin no Jutsu. The wannabe also thinks Danzo may have been the one controlling the 4th Mizukage.
Now,
Danzo can control people, Tobi has most likely controlled someone
Danzo is thought to have controlled the 4th Mizukage, Tobi was Mizukage, if he did this as himself or through control is yet unknown, though I highly suspect it was by control.
Also a side note, Shisui was a master of Shunshin no Jutsu, using it speeds such as never seen before, and now Tobi has his time space ninjutsu which is like a highly advanced Shunshin at first glance, and Danzo has Shisui eye.
Hopefully this is more logical for you.