The value of X is the greatest power among creatures you control when Miming Slime resolves. If you control no creatures at that time, X will be 0, creating a 0/0 Ooze token that will be put into your graveyard as a state-based action (unless something else is raising its toughness).

The power and toughness of the Ooze token is determined when Miming Slime resolves. It won’t change as the greatest power among creatures you control changes.