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