I had a bale of straw seeding mulch with tack https://www.amazon.com/gp/product/B00GR ... UTF8&psc=1
It had been sitting outside in its original bag through the winter. I notice the bottom of it is turning black and, in fact, the 'black' is spreading a bit up the sides of it. Is that harmful to my plants if I use it as a mulch (top dressing)? Or is that 'black' just it decomposing because it got wet and it's not a problem? Is it a fungus or ?