I suspect it's rice and pictures I have found seem to match rice sprouts. If it was another type of fungi or the mycelium was showing any kind of stress, I absolutely would toss the bin. Since this isn't doesn't appear to be a type of infection, I'll just keep an eye on it as there is the potential for it to actually be beneficial to the mycelium.

Heck, this might be something to investigate and experiment with further. While the rice(?) is absolutely using nutrients in the substrate (most definitely nitrogen), there is a very small chance there could be a symbiotic relationship forming here. IF that is the case, the roots may be providing additional micronutrients back to the mycelium. This will take time and a ton of experimentation to see if my hair-brained theory is true.