I've looked around the intertubez, and think this MIGHT be galvanic corrosion, but there was NO sign of it when we bought our 1930's house 5 years ago. The Taco pump WAS replaced 3 years ago (that's when I learned about airlocks, too!)... the old one had no "corrosion" issues, the new one clearly does. Did the plumber miss a step in terms of separating the connecting surfaces with something? I recently replaced the pump servicing the upstairs rooms (again, user error, based on faulty instructions), and the piping on either side of the new pump appears to be starting to corrode as well.
The auto-pressure valve on the cold water inlet to the heating system has never been touched, so why it's doing the same thing is beyond me at this point.
IS this corrosion, and if so, how can it be prevented/cleaned up?