In that case, I still am pretty certain that it is limescale (calcium based). When it gets that thick I have found the cleaners really don't touch it, or it literally takes forever because it has to take one small layer at a time. At the point it is thick like that the fastest removal is mechanical, razor scraping is very effective. After razor scraping a cleaning with an acid finishes it off. Usually a razor blade won't hurt porcelain tile.
You could try citric acid, that can sometimes be pretty effective. The best though is prevention, doing an acid clean more often to keep it from getting built up is key. Abosolute best practice is not letting water sit and dry but most aren't willing to take that step. I used to squeegee my whole shower after every shower, doesn't take long and keeps the shower looking awesome forever. I got lazier and now I just clean the floor/corners in my shower every 2-3 weeks with a good cleaner (varies between a few I have but something acidic that will break the limescale down) and that seems to work as well. I do towel off the shower glass every shower and that keeps it looking new forever. Buildup on glass does damage the surface and eventually looks cloudy and bad even when clean.
If you use an acid cleaner just make sure to wash after with some sort of detergent to neutralize and wash away the acid.
