Do you mean the grey cells?
My understanding is (please correct me if I'm wrong, I respect your vast experience) that bee bread is sealed with a layer of honey - so it glistens. One also often sees dull cells, which are pollen not yet processed into bee bread, which tends to fall out as a powder if the frame is tapped. I am not sure what the grey cells in your photo are, but grey seems an odd colour for propolis. I am keen to learn what an entombed bee bread cell looks like.
This contamination / capping finding is fascinating, thank you for sharing.