your water pump creates flow of fluid by pumping the water.
pressure test your cap and coolant system man.
do you constay get white powder on your reservoir hose?
like do you drive and the. see it, wipe it off, then drive again and more appears?
or did the white powder only appear once?

 

the pressure is to increase the boiling point of the colant so if you have a tiny leak the boiling point may e decreased and your coolant may also be boiling in your engine if you do not have te correct mixture of antifreeze and water.

 

OP, if you're losing coolant (or just have an air pocket in your system) an easy symptom to spot is not getting hot air from your heater. When you start the car in the morning (when the engine is cold) and let it idle for a few minutes, do you get hot air from the heater without revving the motor (no A/C, just the heater core fan)? If not, I suggest you try bleeding your coolant system.

 

i have the exact same problem as with my brother's G35.
I also had white powderly residue on coolant hose. i replaced with new one.
The coolant level still drops over time.

 

The powder on the hose is not an indicator of a bad hose, but rather a leak ONTO the hose where it then dries

It could be radiator cap

It could also be the radiator - the end tanks are not welded on like some quality aftermarket ones are. This can lead to uneven pressure holding them on and therefore cause leaks sometimes. Pressure test - if it doesn't hold pressure you have a leak.