You might also see some occasional green on the surface of copper pipe, often from the water of condensation reacting with the copper. Not to worry about it unless you see a lot of green at a fitting, which could be a very slow leak.
Good info above, look like a residential waste line if so not under pressure which is good. That flux corrosion could cause a slow leak eventually. You may want to just check it every so often. Side note if you are in Puget Sound area your house was probably built in the sixties, that was the brief window we were using copper on residential drain lines. In my experience it has worked well 4 drain lines but you will probably never see it again...EXPENSIVE!