Cupric chloride on pressure relief valve of gas water heater
Wipe it off with a wet rag, then see if it comes back. This could be the result of too much flux being applied before soldering and the excess not wiped off. If you cannot get it off with water, you could try household vinegar on a rag, then wipe all the vinegar off with a wet rag.

The excess flux would have to have been carried down the pipe by water on the outside of the pipe. This could be water condensed from the air on a cold pipe or water from the water heater. If it's from the water heater you will probably find the source if you clean the pipe off and keep it under surveillance.

Where does your overflow pipe go--to the sanitary sewer or to the outside? You could see what happens if you open the PT relief valve by operating it manually with the lever. But only do this if it won't create a mess. Also don't run too much scalding water directly into the sanitary sewer because this might damage the drain piping.

EDIT There is no reason to delay getting the water heater back in service because of the deposits on the TP relief piping.