My parents have a dual (gas) water heater system and there is some kind of build up on the inlet and outlet pipe on one of them (main one used I guess). The build up is around the spot where the pipes go into the water heater and I'm wondering what it is and why is it on the outside of the water heater? I know mineral deposits build up on the inside but not sure why its on the outside. Just to mention my folks are on a well so I'm sure the water is mineral rich.
Most likely caused by diss-similar metals, IE. copper to galvanized piping. You may want to look into installing dielectric fittings to separate. If what you see is a white powdery substance, that is what it most likely is.