After spending several days sitting in the greenhouse, I’ve come up with this oddly pornographic photo 
:
haha. This is percipitate from a 3 gallon bucket of solution that was started with an EC of 1.7 and PH of 6.3 in a closed container subjected to light and warmth for approximately three days. No circulation, stagnant, enclosed container. I’m guessing that precipitant may be similar to what @Howard.Crane has noticed in his system.
I wouldn’t call this “scum”. It is very light and airy and does not adhere to the walls like a bio-film might. Easy to clean. No off smell. It appears to be organic matter that has congealed and precipitated from solution.
Additionally, there was a thin film that had formed on the surface of the solution:
I was expecting this but I could not tell if this film was biological in nature. It looks almost as though it’s mineral in nature and may have formed, in part, due to the noted stratification and PH differences. I don’t know, need thoughts on what this may be. Again, no off smell:
As noted, this tends to confirm what a couple of others may be seeing. In my case, the precipitation did not appear to be a bio-film, it was not slimy to the touch, it did not adhere to the container, it was light and easy to agitate, it was not difficult to clean, no notable off smells formed.
Test completed.