I service a pool on the east coast of Central Florida and the pool bottom has had these types of lines since I started back in December, with no changes over the past season. Chems are always in good range: chl 3-5, pH 7.6 avg, alk 100, ch 250-500, stab 100 (working to bring this down). I saw a worn down pumice stone on the end of a pole a couple of times and wondered if the customers were scraping it on the floor bottom trying to get rid of the marks, perhaps it has contributed??? Here are some pics:
The marks are around various areas of the pool as well, not just confined to the steps. Could it just be normal wear and tear, use over the past 6-7 years?