TXFlyGuy, a couple of months ago I adjusted my autofill to drop the pool level by 1" to see if the skimmer would work better. To my surprise I found a 3/8" wide, grey/white ring around the pool tile at the old water level. The ring had no thickness and strong acid had no effect on it. I tried pummice stone and that wouldn't shift it either. The really annoying thing was that I couldn't see the ring when it was wet so had to wait for it to dry to see if what I had done worked.
Eventually I gave up and adjusted the autofill back up to its previous level.
I don't know if my description of the bathtub ring matches yours, but if anyone has any ideas as to what this deposit might be, they would be much appreciated.