We had a salt water pool

installed earlier this year and I had asked the contractor repeatedly to get the waterline tiles clean before we gave him his final payment. He said he had it cleaned and unfortunateunfortunately he got his payment before I verified...dumb mistake! I have tried all I can think of that is reasonable to get the tile clean but this stuff just will not come off! I have tried scrub brushes, magic erasers, running razor blades over the tile, fine grit sand paper...I am worried I will damage the tile before I figure out how to get this stuff off. My best guess this is mastic or plaster or maybe concrete from the coping? Anyone out there have any ideas??