I have a maple tree that lost a large limb (one of three main limbs of the tree) in a strong storm recently. When the limb was cut, the wood was dark in the center and looked stained - like a water stain - over the center 75% of the wood. The sapwood around the edges looked fine. Now that it has been cut a week, some mold is popping up at the edges of the stain pattern. Anyone seen this before? Do you know what exactly this is?

Thanks.