Green rust on water heater discharge pipe, what cause it?
Copper oxides = Green &quot;rust&quot;
Water causes that, can be aggravated by failure to clean acid flux from soldered fittings, but since yours starts right up by the threads I'd guess you have some &quot;weeping&quot; out the valve and the threads are none too watertight (sloppy workmanship - &quot;just a drain, no need for it to hold water, never going to be used anyway...&quot;)
Weeping can be caused by a failing valve, or by a lack of expansion tank combined with a check valve or thing that acts like a check valve (such as a pressure reducing valve or PRV) causing the pressure to rise when the tank is full of cold water, no water is used, and the cold water in the tank is heated and expands.
At 20 years old, have you changed the anode and flushed the bottom regularly, or just let it sit?  You might be reaching end of life, if you have not maintained it. The oxides on the drain line don't indicate one way or the other on that, really, but it's worth thinking about anyway.