I did this home inspection a while ago that left no doubt that this was Termites & mold. There were literally thousands of termite tubes dangling from the floor and joists under the home. There was so much damage to the joists, I could easily put my hand all the way through some of them.
Near the top of the photograph is a termite tube, or shelter heading toward the ground and it is surrounded by some type of mold growth. The white on the floor joists and sub-flooring is also some type of mold.
This is not a healthy environment for anyone to live in or near!