fish may have been overfed and the white stuff was mould that had grown on the excess food on the substrate
did you do a gravel vac as well as water change as the gravel under the white stuff may have become anaerobic
just a thought
talked to my beardeds,
the girl said she likes her hide for time out times and when she is feeling insecure
the male said he hasn't got time for a hide as it would mean less time for people to look at him as he looks so good