haha. I thought about that upon seeing these mushrooms tbh.
I found out what these rather non-magical shrooms are though. I believe they are known as the plant pot dapperling.
I suspect that wood barrel has a role to play. At first, I thought it was due to all the organic rich amendments, but then, I noticed a rose plant in a nearby wood barrel sprouting the same types of mushrooms. Both have been mulched with alfalfa.
From my research, they don't necessarily cause any harm or benefit, and are eating the decomposing topsoil and material. But yes, I have used this soil in other fabric pots and they don't sprout this type of mushroom, however I have seen the white mushrooms in fabric pots.