It was already wet when they installed it.
I’ve see it many times. Trusses on the wet ground.
Oh good grief! I hear you. We do testing if it’s requested or recommend testing if it’s necessary but we never say “mold” unless we have the lab results in our hot little hands! 
efflorescence
I know what it isn’t, and that’s efflorescence, that is unless that wood was treated with salt water. 
From here in Ohio it looks like old faded paint.
If you “think” it’s mold, write it up as “an organic growth consistent with mold was viewed on the joists”, and recommend testing and mitigation if needed.
There’s probably some chemistry that can make that happen (strictly speaking, a salt isn’t NaCl, just ionically neutral, yes?)
If you were inspecting my home you’d call out weird things all the time. For example, those joists? That’s how I clean my paintbrush if I’m doing something in the area
Also, I don’t think they’re mineral deposits. It’s really puzzling why it’s only on one side of the joist. It could be accumulated dust over time sticking to the wood fibers. Or the leftover remnant of whatever that is sealing between the joist spaces.
If it’s mold, I’d be so disappointed.