Rock salt quality is partially a regional issue and a QC issue. Salt is very heavy and expensive to transport so rock salt in one region of the country may be dirty, while oters may be very clean. Salt is rarely transported long distances. For the most part, salt is salt. I use rock salt, but our sources are very clean here in California. I have been to parts of the country where the rock salt will leave dirt residue after only a few bags. I would avoid salt that is dirty. If you do use dirty salt, be sure to clean and sanitize your brine tank annually. Let the brine tank run almost empty of salt, then clean it, add a 5 gallons of water, fill it with salt, and regenerate the system. it might not be a bad time to sanitize the resin bed as well.
FYI, I used to sell millions of pounds of salt a year, we only used solar rock salt. Again, our source was clean, so it was not a big deal.