Tom said:That's not shell rot. Shell rot typically occurs on the plastron. Sulcatas almost never get shell rot.
Yours looks like it has calcium powder residue or hard water stains.
Tom said:I collect rainwater, filter it and store it in 5 gallon water jugs. I use this water for daily shell sprayings and I rinse them with it after their soaks in regular water. I suppose you could use distilled water the same way. Rinsing them with mineral free water, keeps the hard water stains from forming. But don't use distilled for drinking or soaking.