that is something i've not seen.
my first guess would be crystalized honey (or sugar if you are feeding sugar), as they often cap over the top of beebread with honey.
i'd taste it. if it doesn't taste like sugar/honey and no one comes up with a satisfactory answer based on the photo here, i'll ask around.
beehopper, don't forget that yeast is only active in beebread for the first 24 hours or so after being collected...after that the medium becomes too acidic for yeasts and molds.