I've seen this happen, but only occasionally been able to find out why.

Right now, I've got a commercial kiddie pool (6,000 gallons) that turns milky and fizzes every other time when I add salt, which we do a LOT, because of all the splash out. Dunno why, but I don't worry about it.

I've also seen it happen, when air gets dissolved in your pool water. That happens some times, if you have a suction leak that brings a SMALL amount of air into the pump. When this is the case, there will usually be a film of bubbles on the sides of the pool, so that if you run your hand on the pool wall, it will release the bubbles. Usually, you can feel them, too -- makes the pool wall feel slick.

But, I don't have any general explanation.