Pro heater trips out on overheat?

[Larry B]

 Decided to check and see if pro heater still functions proper. Tripped out on overheat due to poor circulation by feeling outlet hose temp. Upon removal of unit it was found both the inlet and outlet fitting (brass) to be plugged with some sort of hard green stuff. It was evident why unit trips out on overheat. The picture below shows the plug and corrosion. Brass often turns green in presence of corrosive chemicals. I have since drained all antifreeze and flushed with water. Engine and heating system.  Fill and drain -several times. The old antifreeze is not clear any more, it has turned cloudy with slate grey tinge. Gone to recycle. What is going on and why???. I have also removed other brass fittings on the engine looking for corrosion or build up,, none found so far. The heater has not been used in a couple of years so the plug may have been there before engine overhaul. I would like to get to the cause of this incident and correct.  All guidance and help gladly accepted. 
          Larry B

[Larry B]

  Sorry same photo twice. In this photo you can see a hole eaten through the end where hose clamps on.
  Larry B