Moisture and Removal
Mushrooms thrive where moisture is prevalent, and the only sure-fire way to get rid of mushrooms in a shower is to manually remove them and let the shower area dry. For example, after each use, allow air to flow into the shower area.
Use a fan or turn on the bathroom's exhaust fan to help this process.
Cleaning and Sealing
Clean your shower regularly. Use a pH-neutral tile cleaner and a fungicide capable of killing mushrooms.
Allow the area to dry and apply a high-quality penetrating sealer to grout and shower tile. Sealed tile is far less likely to allow mushroom growth.
A last-ditch DIY solution to mushrooms growing in a tile shower involves removing the grout and replacing it with an epoxy-based grout. Epoxy grouts inhibit the growth of mildew and other types of fungus (like mushrooms) that thrive in moist, dark areas.