Planks under metal roofing

Can anyone tell me what has caused the wood fibers in the roof decking planks to be heavily furring. It was on several of the planks and pretty heavy on some. There was no elevation in moisture on the planks but there were some signs of past leakage. This house was an older home that had been moved to its current location. Typically I don’t touch something that looks this was as usually I think of growth/mold possibly. But I did touch this and it appeared to be the wood fibers and possibly rot.

Just recommend testing for mold to start.

You said the building was moved from another location. Could the building have been exposed to salt water spray like near the ocean?