Any ice that forms from water will be free of salt -- the process of crystallization does an excellent job of removing impurities.
But that's only when freezing out of liquid water, which is not a process that happens today at the Martian surface. When ice forms out of the air, it forms as tiny crystals -- "hoar frost" we call it on Earth -- and forms on substrates, such as grains of sand. Since the soil of Mars is laden with salts of various sorts (including things a lot nastier than NaCl, such as perchlorate salts), it's hard to believe that the Martian hoar frost wouldn't be contaminated with salty sand and dust.
And, of course, the Martian atmosphere has dust suspended in it which is continually precipitating out and dust which is blown up by the not-infrequent dust storms and dust devils and all of this falls on any newly-formed frost.
It's very hard to see how there could be any macroscopic ice deposits on Mars that don't have dust and salt as impurities.