I have had mine do this a few times. It was because the were getting used to the tank one time and the other time is because my eel knocked their rock over and it landed upside down. Each time they were a little shriveled up for a day or so but bounced back. I would check to make sure you tank parameters ( like salinity, ammonia, nitrates etc) are ok and if everything is fine then give it a couple of days. If something is off on your parameters then you need to fix that and see if the mushroom gets better