yellowing of shower stall fiberglass floor

It's a fiberglass shower right?? Mix Whisk 2X HE laundry liquid soap with 20 Mule Team Borax ...into a paste (don't use water)...take a damp sponge and apply to the floor and walls. Let it sit for a few hours (overnight is okay, too). Then hose it down and see what it looks like. Might have to do this once a week to get it looking nice again. This is just part of my regular routine.
Works in the bathroom sink, too.

Watch that floor... it might be slippery !!

Paint is probably the best way to get sun damaged plastic back to it's original color. Might be a good idea to add a blind to the skylight to prevent it from happening again.

Try Barkeepers Friend