What's growing in there in a saprophytic fungi, a decomposer. The mycorrhizae don't grow without live plant roots.Yeah, it has (microhazea (sp)), thought it was that, but never seen to this extreme.
Yep, the fungi use a lot of oxygen when they're growing fast like that. I've attempted growing mushrooms & plants in the same container a few times with mixed results. Agaricus blazei grew great with cannabis plants, been wanting to try that again. The biggest issue with doing this is Fungus gnats, they get bad quick, lol.IME, those bags that had the biology kick off hard in the bag are GREAT for seedlings. When it kicks off in the pot, the oxygen demand is high, and they'll struggle just like overwatering.
Nice score!