Even brand new unopened saltmix contains some moisture.
If you get a lot of additional moisture in an open bag of saltmix, calcium carbonate can form as a solid within the saltmix powder.
If you go to use this bag of salt, and find that some of it just will not dissolve not matter how long you let it set, or how hard you stir it around .. then chances are very good that the stuff in the bottom of the mixing container is calcium carbonate ( chalk ).
I sometimes see a little of this in my mixing containers.
If I saw a lot of it .. I would probably hesitate using it in my reef tank.
Best thing to do as mentioned is buy the large buckets of saltmix.
Or if you like buying the smaller bags, afater you open the bag pour the unused saltmix in an airtight rubbermaid type storage container.