Rock salt is DIRTY!
Sea salt isn't much better and there are other sea minerals in it and worse yet, the countries that make sea salt are all third world (vietnam, ...) and a lot of those countries dump raw sewage in the ocean in same area they get the sea water to evaporate to get the salt.
Himalayan salt is the purest on Earth because those deposits have existed for millions of years - no pollution, nothing BUT the pink is other minerals that are actually good for you so its kind of "contaminated" from a sodium chloride perspective.
Salt you buy in the grocery store isn't super pure either. A lot of salt has an anticake agent added which wouldn't be wise to put in a colloidal gold cell. I've even seen that in some kosher salts.
Really, the only salt that would be completely pure would be something thats labeled as USP grade and I can't say I've ever seen any salt from a grocery store thats been labeled as such. Doesn't mean there isn't any - just have never seen it.
Going bad means it starts smelling funny or evolving life, usually. And yeah, it does which is why the formulas only make about 250ml. Keep it in the fridge and endeavor to use it ALL up within a week or two and then make more. Easy if you use the salt lake minerals 1% gold chloride method. Not as easy if you're making it using electrolysis.
I'm not sure what gold flakes rising to the surface means. Perhaps someone else can chime in on this one.