During my salt test, I left every salt tested to mix for at least one week. Some went 2 weeks depending on how lazy/busy I was before using it for a water change.
Off the top of my head I would say ESV, HW Reefer, and NYOS were the cleanest. Tropic Marin will usually leave a dark green ring around the waterline which, after talking to Hans Werner on here, I believe to be an iron/manganese precipitation. Scrubs right off and poses no threat to anything. A few salts left this dark green ring but I couldn't name them all. It is odd though because many new salts contain elevated iron and manganese.
The reasons for the "crud" left behind can be quite mysterious for some salts.
Avoid any high alkalinity salts like Red Sea Coral Pro, AquaForest Reef Salt Plus, Reef Crystals, etc. I would also avoid Red Sea Blue Bucket, Instant Ocean Purple, Brightwell, Fritz, and any salt with pre/probiotics.