Blue/Green oxide on pipe feeding HVAC unit

[AJTSheppard]

I have a build up of blue/green oxide (more blue than green) on a copper pipe that feeds a central air HVAC unit. There are two copper pipes feeding the unit, but only one exhibits this problem. Even though the worst of it was easily removed with soap and water, I would like to know what causes this and what can be done to make it go away.

Before cleaning with soap and water ... see photo, and after cleaning ... see photo.