Can anyone tell me what this white stuff is on the very bottom and on both sides of my hot tub flagstone overflow and on the waterline tile directly below it. This is the only place it is on the entire pool. All my numbers are are of good value. I tried rubbing it with a pumice stone before I took this pic. I wouldn't think it is scale as all my values have been spot on since my pool has been built. This is a new pool of about 3 months.