If you must know, Otogakure is the village that Orochimaru created by himself, and tried to crush Konoha several times.
Mitsuki seemed like a normal student, until one moment where he was shown communicating with someone.
Mitsuki was seen talking to someone via some snakes at the end of episode 5, which had made a lot of fans suspicious of his activities. This suspicion did not stop.
At the end of Episode 7, again, he was atop the Hokage rock, and he was communicating with someone. Mitsuki reported the information about the “Dark Chakra”, and the events of the village.