It sounds like you are describing efflorescence or a mineral deposit. It would be easier if you send a photo. If it is at the water line it's most likely calcium deposits. If it's at the edge of the pool it's most likely due to water seeping out. (efflorescence)
white white white
limestone has a white streak.
White.
Limestone can and does appear in shades of green, due to impurities.
Limestone with a pure white color normally formed in clear, warm water. It could also have formed in a river estuary as well as river beds.
When limestone (CaCO3) is heated, calcium oxide is produced!
Sand it down and paint it.
Possibly over time, but nothing you should notice for as long as you own the pool. You will however see a white residue on your nat. stone. This is caused by splash out. The water on the pool deck will evaporate, but the salt will not. This is easily cleaned up by hosing off the spots with tap water. If you do not clean them off in a timely fasion, it may cause excessive build up, which will have to be acid washed off.
Limestone can come in grey, yellow, white and a bunch of other colors too.
Calcite.
Chalk.
Polished white limestone.