When you take a shower, the hot water and soap do a great job of removing oils and perspiration from your skin. When you step out, feeling clean and refreshed, that soap you used is back in the shower, mixing with the oils from your body to create a mess on the shower floor. Over time, especially in fiberglass showers, the soap scum/body oils have a tendency to build up, layer upon layer, until the grime is caked on. Though every professional house cleaner has his or her own special recipe to remove this black, greasy grime off the shower floor, only some can be relied upon consistently.
Dampen your sponge using water from the sink. Wring it out very well so that it doesn't drip water.
Pour a handful of detergent onto the shower floor in the corner.
Scrub the pile of detergent in a circular motion. It will spread out as you scrub, so keep moving to a new area.
Add more detergent as you scrub your way around the bottom of the shower. Try not to add too much water as it will interfere with the mild abrasive action of the detergent.
Rinse the shower stall well to avoid a slippery mess from the detergent.