I'm an IO user and don't have this problem. Do you have box or bucket? If bucket, make sure you roll the bucket to mix up the salt each time. The box comes in smaller size bags which mix easy.
I never heard of rolling the bucket to mix the dry salt. I'll try that next time. I only order by the bucket and mix in a spare tank I have with a power head. When emptied I saw lots of white residue.
I get a type of film in my Brute mixing can also using IO Reef Crystals. It looks white but when you clean it out, it looks more yellow. I have a Mag 7 pump that is constantly mixing so I don't think it is a lack of flow. I have to completely clean it out every other month??