We have a 4 year old Ruud gas furnace and I noticed today that there is white, flakey material coming from the joints on the exhaust pipe.
What is this?

I think it is zinc oxide carried out by condensation. I am asking to have you question moved to Heating and Air Conditioning. Trust an answer from Northern Heat or Ttop.

It is normal, just impurities in the gas that condense with the flue gas.