I do not think a pair of Sharingan are seeing through Mizukage's Genjutsu.
- Genjutsu affects the brains nervous system. The Sharingan can recognize Genjutsu because they can see chakra flow so someone with an irregular chakra flow would be under Genjutsu. Since the eyes are connected to the optic nerves in the brain, this lets them see through Genjutsu.
- Troll Kage is not putting people under a Genjutsu that is affecting their nervous system. Gaara, Onoki, Temari, all shinobi etc, did not have irregular chakra flows. Their nervousness system was not affected at all because it is not such a Genjutsu. They never needed to be "released" from the technique.
-Troll Kage's real body can't be sensed during his mirage, except with contact sensing. Only after the mirage was over, were sensors
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, meaning during his mirage either he and his clam must have the same chakra because its impossible to tell the difference between them, even with sensors, or during his mirage, he and his clam have no chakra to even be sensed to begin with. I don't know which is true but if either or are true, Itachi can't see through it.
Its 50/50 for me.