Oxidation caused by leftover flux on the pipe joint. Take a wet rag and wipe off what you can, steel wool will also help remove it. It's not really a problem unless there is large amounts of the gunk. Most experienced plumbers wipe the freshly soldered joints after soldering them.
Oxidation of copper will produce the blue green patina cupreous oxide. It is rust only on copper. Clean it with steel wool or a fine grit sand paper an application of a light machine oil wiped on then off should reduce a chance of recurrence.
Leftover flux on the joints oxidizes the tubing in a moist hot environment. Worry about important things.
