Just a noob myself, but have had problems with reef crystals mix sitting, too. My bucket wasn't covered, but after about a week, I started to notice a film of stuff on top. I tried to remove it with a fine net and it broke up and spread through the mix. The mix was heated to about 78, and a powerhead was running the whole time. I didn't give it much thought until I saw this post, but concerned me enough to toss 7g worth of crystals, and 8g(evaporation) of distilled water at just under a dollar a gallon. I would think logically, that having it sit around may allow things to grow and not be as good as mixing it fresh. I just make mine a day or two before I plan to use it to let it aerate and dissolve crystals thoroughly.
That being said, I'm just a noob so I'm very curious to see what other people say. I've had extra mix after a wc once, and put it in a gallon jug sealed. I didn't keep it for long enough to learn anything, but I could see it being nice to have a bit of water ready for emergencies if it could keep long term in an airtight container. Pickled foods last nearly forever, sw seems similar. Maybe if you added some bleach to it... haha just kidding! (Horrible idea, but someone braver than me might say with a strong dose of prime right before using it that this could make your mix last almost indefinitely)