It would take a heck of a lot of magnesium to scale out. Typical hard water has both calcium and magnesium, but the solubility of magnesium carbonate is far higher than that of calcium carbonate. As for SWCG pools, the main issue is with splash-out of magnesium sulfate that has a high re-crystallization pressure so can do the most damage to unsealed soft stone or concrete. Even so, if the magnesium were a minor component of the ice salt, I wouldn't worry about it.