Mushrooms yes mushrooms can grow when very contaminated water infiltrates areas of your home it is not supposed to. Such as a sewage back up. If you look closely you will see in the 1st image on the left what looks like black dots. Those are actual mushrooms. Contaminated water can not only be a safety risk but cause serious damage. Even after just two days fungus can develop and pose problems. This was a sewage overflow from an upstairs toilet, that the client thought they got all cleaned up. Two days later mushrooms sprouted from the dining room ceiling. It is never wise to handle a water loss for yourself. Placing your hand on the wall to see if something is dry is not scientific and can be very harmful.