I've gotten that stuff in several of my beers. I can't figure it out either. I've gone way detailed in sanitizing, and it's helped a little. However, even with that stuff in there, when siphoning it out, the majority of it will cling to the side walls, and I leave about .5-1" of beer in the fermenter, and ie never had any of flavors from it.
Still, I'd like to know what it is!