Moisture. I wouldnt worry about it unless its excessive after the car has warmed up. You say it gets worse as it warms up, but does it continue after the car gets to operating temperature?
Check the oil and coolant see see if there is a mix of the two. The oil will have a salmon/orangey color if coolant is mixing in. You would be able to see some oil in the overflow/from the radiator fill neck if its in the coolant.