The Queen is, 'Your Majesty,' the first time and, 'Ma'am,' subsequently (all but her family use this form of address at all times). Her father was, 'Your Majesty/Sir.'
Scottish Kings were, 'Your Grace,' then, 'My Liege /Sire'.
An Emperor would technically be, 'Your Imperial Majesty,' an Imperial Highness would be his eldest son, the Heir Apparent, but your kingdom may have different protocols.
The Emperor will have decided his own title, and while he may tolerate the uneducated using a different form of address, he will expect this at all times, and anyone entering his presence will be instructed in the correct form.
There will be no variation, so I suggest, 'Your Imperial Majesty,' on the first occasion, and, 'Sire,' OR, 'My Liege,' subsequently - NOBODY shortens the address in any circumstances and outright flattery is rarely tolerated.