I do not know the answer to your question but I hope someone can answer. I have had that issue with my toilet seat. I needed to use the toilet when i was coloring my hair. When I got up from the seat, I turned to flush and not having seen it drop, a "plop" of red got onto the seat which I have been unable to remove with vinegar, peroxid or even later clorox.
Mine is not as serious as yours. Mine is a removable toilet seat but I still wish I did not have to change the darn thing out. I hope you get the right answer which will work for your shower stall.