Well, in the past I have actually grown large amounts of mushrooms on coco, so it's not surprise to me that you caught some mycelium on there. I live in a desert, so I would never run across this but if you live somewhere where mushrooms grow in the wild like a humid climate, then some mushroom spores just found their way into your grow and colonized your coco. Personally, I wouldn't worry and I would just let them grow out depending how far along the grow. If you're just getting them started it will need to be addressed, unfortunately I know of nothing you could safely spray on your plants medium that would kill the mycelium without damaging the plants themselves.