GQ TB42 runs well but has one corroded sparkplug

[mudski]

Thanks!
That was my first thought as well. But the almost no coolant use ... ?
Would even a teeny weeny bit of coolant in the one pot cause that even though it runs smooth ?

It could just be leaking into that pot when the engine cools, then the warmth in the pot "steams' away the coolant giving the spark plug a corroded coating. Corrosion needs moisture to happen.