water Fert/Water question

I keep two 1 gallon jugs in the grow room, one simple purified water and the other the same, but with a quarter teaspoon of CNS-17 3-1-2 solution.

I'm getting the idea that storing water that's loaded, even just slightly, might not be the best. The inside of the jug with the CNS is a light shade of green and the water feels "slick" between my fingers. I'm obviously growing a culture in there.

So it's best to mix what I need, when I need it, or is a little biological matter ok for watering?

I only mix as needed.
 
If premixing, use a vessel that blocks light 

Yes, second that.  It can be used in hydroponics, so it has the ability to be premixed.  But I'd recommend agitation, on top of light blocking containers.
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I always mix 2 gallons at a time.  For that purpose, I bought a 1 oz. hand pump that fits the bottle. (amazon.com)  One squirt does the deal.
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I am doing the same thing and have observed the very same thing. For me, that green growth is not that problematic, because I know it's just an algae growth. They are attached to the jug and stay there, water inside is still colorless, without funky smell and without strange viscosity. I use rain water to prepare the fertilizer mixture, because I also have carnivore plants and I have to collect rain water anyway. When I'm low on water, I use tap water though.
In 3 years or so, I have noticed small amounts of the same algae growth in my rain water jugs as well. Like I said, there's no harm and water is still clean inside, perhaps even cleaner, if the algae take away some nutes from it.
 
ps: I have left rain water in those jugs for more than 5 months, and there were no foul smells coming out when I used it. I would not be too concerned about it.