Can you reveal Tideturner from hidden after Charm resolves and moves your only unit (Mundo) off the battlefield, before you lose control of that battlefield?

Ruling: No, you cannot reveal Tideturner after Charm resolves. When Charm moves Mundo off the battlefield and he was your only unit there, you immediately lose control of the battlefield (since it's not contested and you have no units there). The hidden Tideturner is then removed during the cleanup that occurs from the movement.

Sequence:

• Charm resolves and moves Mundo to a different battlefield
• The movement triggers a cleanup
• During cleanup, hidden cards are removed from battlefields not controlled by the same player
• Since you no longer control the battlefield (no units, not contested), the hidden Tideturner is trashed
• You cannot reveal it before this happens

Nuances:

• You could have played Tideturner from hidden BEFORE Charm resolved (during the showdown when Charm was played), even if it would have no legal target
• Hidden cards are removed during cleanup, not immediately when you lose control of a battlefield
• You can maintain hidden cards at a battlefield you don't control if no cleanup occurs, though this is practically impossible in most game situations