Probably a member of Lepiota - the isopods should in theory eat it and get rid of it. Keep in mind plucking it just removes the fruiting body - there's still the mycelium, the roots of the fungus in the soil so it'll keep sending up mushrooms. It probably just indicates you have a lot of organic matter in that potting soil, which I guess is good, since it suggests there's no fungicides or dangerous stuff?