Phantom Power is a 48v power supply for condenser microphones. The power is sent through the XLR mic cable. However, the KPA doesn’t need this power. There should be a safety cut off to stop it damaging the KPA but it is always better not to risk it by just turning phantom off at the mixer.
The PA has lower output than your monitor speaker so you would be better to leave all the available power to handle vocals rather than sap this with guitar.