Pet safe salt is Magnesium Chloride. I would not want my ducks eating the solid chunks for two reasons. One, it generates heat when it encounters water and can be irritating to skin. Two, it is a laxative. In fact, it is what the doctor has you drink before a colonoscopy to “empty you out”. Ducks with diarrhea sounds like a bad idea to me!
If you are only scattering some on ice, and it melts the ice and the water dissolves it before the ducks encounter it, I would not expect problems.