That stuff is the ferment culture. How to determine if your batch is all "good stuff" or contains some "undesirable" elements is unclear to me. What your cture contains is determined in part by what the stuff you're fermenting "came with" in terms of spores & stuff, and also what is in your air. I PRESUME I'm growing "good stuff" as my ferment culture ... but have been reading up a bit about fermenting cereal grains for increased nutrition and maximum "safety" (least amount of "undesirables" in the culture I cultivate). Inasmuch as fermenting cereal grains for increased nutrition is commonplace if not necessary in certain parts of the world, and inasmuch as low-tech solutions are the most practical, I've been reading about comparisons between "natural" fermentation (no starter used) and cultured fermentation (where specific starters are used) ... It seems better results (safer and more nutritious) are possible when starters are used. Mostly I'm reading out of curiosity, but did find a cool study about the increases of protein in cereal grains after various ferments.