Layer of yeast on top of sourdough discard? Most of the time when I feed my sourdough starter I find uses for the discard, but on occasion I don't have time or good ideas, so I put it in a separate jar and store it, with a tight lid, in the fridge.
Yesterday I took the discard jar out and found something I've never seen before: a layer of dry, off-white, stringy material (see photo), with the distinct smell of baker's yeast: 
Hooch often appears in the discard jar after a few days, but this is definitely not that. It also doesn't look like any type of mold I've ever seen - and definitely doesn't smell like it either.
Luckily this is just the discard jar and not my active starter, but I'm really curious. Could this be yeast? If so - where could they have come from? And how have they grown on top of a lively sourdough culture?
N.B.: I had made this starter from scratch (i.e. flours and water) without using yeast or any other additives, and have never seen this in the 6 months I've been using this culture
Sourdough culture is partly yeast.  Since this container was made up of sourdough taken out of your primary culturing vessel, and not fed, that is probably dead yeast that floated to the top. After all, it hasn't had any source of sugar since you put it in the fridge.