Clean the problem area with equal parts bleach and water. Scrub with a scrub brush to remove any soap scum or mildew. Rinse with plain water and dry thoroughly before proceeding.
Insert a tube of epoxy crack filler that is designed for concrete into a caulking gun. Cut off the tip and insert a nail into the head to release the epoxy.
Press the tube's nose into the start of the crack and depress the plunger to fill it with epoxy. Make sure the epoxy goes all the way to the back of the crack. Move along the crack with the epoxy until the whole crack is full.
Wet a finger or rag and run it over the epoxy while it is still wet to smooth the epoxy flush with the shower wall. The epoxy won't stick to a wet surface.
Allow the epoxy to dry for at least 12 hours, or according to the manufacturer's instructions, before using the shower.