Unfortunately, no closer pictures. Once I realized it wasn't the neem, I started getting rid of it and there's nothing to take pictures of right now. (Fingers crossed that it stays that way. It has been a few iterations of scraping it off. )
That slime mold suggestion seems spot on, when I looked that stuff up. Like tiny crystals, almost.
They are all in the same mix. Part compost, part potting soil, which does have fertilizer in it. The pots themselves were the only difference. They were all in those plantable pots, but some of those were in those nice breathable brown round ones and when I ran out of those, I put the pots in the cheap plastic ones. The cheap plastic ones were the only ones to grow the stuff.
There is no fertilizer in the water, and usually I just use tap water though sometimes I get it from the rain barrel.
Thanks!