Leave them or remove them. They're natural. "Mushrooms" are the very transient fruiting bodies of fungi that normally stay dispersed thinly in the dirt/soil. When conditions are right, and that's usually a very small time frame, they grow the big parts to try to spore off. MG is really no different than any other product of its ilk. It's made and mixed on the ground to contract specs by recycling and composting companies all over, and such "contamination" is expected.

That is a very strange photo.

I wouldn't fear a thing from them. Just ignore.

I have about four dozen baptesia seedlings in a commercial potting mix. A couple of them are looking yellow, which initially surprised me, as baptesia is a nitrogen fixer. Well, it is a nitrogen fixer when paired with the proper soil microbes and fungi. That commercial potting mix may be lacking in the partner organisms. I'll be re-potting and will make sure to mix some garden soil into the pots, because plants do not need a sterile environment. Ideally they will have a soil that includes all kinds of living things.

MG is EXTREMELY water-retentive right from the bag, so mixing garden or topsoil into it is not a good idea because it can only ADD to its tendency to hold excessive amounts of water. Also, soil partnerships aren't nearly as beneficial or important in containers as they are in the open ground. Container media suitability is pivotal on the soil's structure, not its ability to feed the plant. Get the structure right and you have something to build on; get it wrong and you'll be constantly fighting against your chosen soil for control of your plants' vitality, which unfortunately is usually the case when using MG's soils.

Growing in pots is much closer to hydroponics than it is to growing in the earth, which is one reason why trying to move the garden into a pot works so poorly. Taking and maintaining control over your plants nutrient supply is monkey easy, so it makes no sense to sacrifice soil structure on the altar of nutrition.

Al