What causes this type of staining?

[rs03]

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?

Thanks much.

 

[PoolGuyNJ]

The last picture showed it. It appears to be an area that the plaster mix, when applied, was either too dry or had been sitting too long before it was applied or were hit with wind and the wind dried the plaster a bit faster, causing minor shrinkage cracking. They are permanent. They are also harmless normally as long as they don't penetrate. After 7 years, it there were penetration, it would have likely blistered and delaminated. It hasn't so until the finish is redone, they will have to live with it.

Scott

 

[rs03]

Thank you so much Scott.