When I moved into my house, the back wall of my tub enclosure had what looked like black handprints all over it! No idea what caused it, suspected it was mold coming thru from the other side. Tried for 8 years to get it off -- every cleaner on planet Earth, steam, scraper, all to no avail. Then I tried Awesome, a liquid cleaner that many on here recommended. You can find it at the Dollar Store.
I wet down the wall, waited about ten minutes, and was able to easily wipe the marks off with a soft scrub!!! EIGHT STINKIN YEARS and I couldn't get that stuff off.