My vote is on calcium scale.
Pool School - Calcium Scaling
Since it seems to be on tile, I'd try to brush it out using diluted MA and see if that helps. And keep your CSI between 0--0.3 (see Pool Math).
Yippee
It has in the past but to be honest, I don't recall exactly when.Did the pool fill up recently due to rain?Sent from my iPhone using Tapatalk
Often times you see that kind of ring when a pool fills and then is allowed to evaporate to its normal level. The result is a constant deposition of on the tile and a white "bath tub" ring.
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Oh man, that stinks! I hate those things....would not plant one anywhere in my yard.
For those not familiar - 99.9% of the time, when an agave sends up its shoot, it immediately dies after the flower(s) open and are spent....it's truly an annoying plant because there's no easy way to predict when it will happen and then you have this big giant dead succulent to deal with.
I say plant some smaller cacti there.
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by the way, love your ocotillo in the background. they are beautiful!