When IGBT is off what is the leakage current between collector and emiter?
Ices is the leakage current between collector and emitter at the specified Vces of 900V. It is different at 25C (10uA) and 150C (150uA) junction temperatures.

BVces is the breakdown voltage between collector and emitter. 'Breakdown' is defined as the voltage at which the leakage current rises to 250uA, at 25C.

Please don't put these voltages and currents into 'Ohm's Law' ratios. It doesn't really help. There is no 'resistor' in the device, and any calculated 'resistances' will be far from constant, and vary wildly with voltage and temperature. That is, a resistor model of this type of device is not generally useful as a 'current source' model for the leakage current.