Definitely not mold. If you do a search, you'll find this has come up a number of times and has been answered by me and others. Can be very disheartening when it happens. Oxalic acid (as Bruce says) is a good remedy, as long as there isn't a piece of iron/steel buried in leather, possibly from a band knife during a splitting operation. Takes a little longer to show through than when it is on the surface. Pattern was traced on and all the swivel knife work was done when it showed up the next morning. It was in a 1" wide border area, else I may have been able to hide it with some tooling and dye or paint, especially if it was in the back grounding.
Search for blue, black gray spots. It's the same process as vinegaroon.