Here is the picture, https://i.imgur.com/fIlqAk2.jpg and https://i.imgur.com/cFu9SxX.jpg. We recently bought a house, built in ’77. It has been a box of surprises to be sure. We found this when ripping up the old mdf flooring in one of the rooms. We cleaned it, applied Kilz to the whole floor, and put down new flooring. Fast forward a month, and I had to pull up a small section, and noticed water, so reoccurring issue it is. Is this mold? And where is it coming from? It’s either leaking in from the exterior of the house, and making this pattern in the small spaces under the flooring, or it’s coming in through the slab the house is built on. Google has let me down and I can find nothing image wise or description wise that resembles this. Help?