We found this is it mold?
That appears to just be the broken down remains of rubberized carpet padding. If you look on the tack strip (the little strip with the nails in it that holds the carpet in place), there appear to be larger pieces of it still embedded on the strip, probably from a previous carpet installation that was removed sloppilly years ago. That's not to say you don't have mold somewhere, but that's not mold.