Our pool has Barbados Blue Quartzs plaster put in 17 days ago, everything has been great and have been monitoring the chemical since day one. This last weekend we went to the coast for 2 days. I checked the water balance before and fixed it to be good. I got home today and found this in the Cabo(sun) shelf. Some grey mottling to the looks of it. any idea of what this could be and how to fix it? It's the greyish crescent shape around the bubbler. the dark spot near the bottom of the phot is my shadow.

