Propane Fitting Corrosion

[Mark in Arkansas]

I’m new to the forum. I have been lurking on/off for years.
I have an emergency heat propane fireplace. Hasn’t been used since it was installed 8 years ago. I just opened the access panel to the shutoff beside the fireplace and noticed this corrosion? The pipe coming to the fireplace has a shutoff at the other end of the run and has been off since installation/testing. Any thoughts on what might have caused this. The black pipe thru the floor is connected to CSST in the crawl space. Not sure if that matters. Any thoughts?

 

[Fitter30]

It has to be moisture. Since it's behind a access door it in a uncondition space. Humidity travels from high to low and heat the same way id check the temperature in that space next winter.