It's under my car between drive and passenger seats.

Not sure if it's from AC, but I notice it after driving on the highway and stopping for gas. Been doing heavy driving in the hot north east.

Also, AC is pumping out white cold air when blasting.

Any ideas on what the cause could be?

 

Probably water from the AC evaporator.

 

In very humid weather the ducts may blow out frosty air. The AC removes moisture from the air, dry cold air in a hot humid environment will blow frosty air. Like opening the freezer at home.