Can i turn off the IMO when the external clock is used to save power ?
The IMO is turned off in sleep -
Though you can disable IMO, when using external clock by writing a '1' to the IMODIS bit in OSC_CR2 (refer TRM). It is not recommended, as disabling the IMO can cause flash read corruption over time and temperature. So the only recommended state to disable IMO is sleep state (which by the way automatically happens).