I have also encountered mould growing on shellac discs; apparently they were packed (stored) in hay, prior to being donated. It was a white mould (hay mould), and did etch into the disc (feathery patterns).
A related question: I have, in the past, encountered a very few shellac discs, usually stored under abysmal conditions before I acquired them, which look like they have actually been attacked by mould and have the resulting permanent damage. Given that shellac is an organic material, is this possible? Steven C. Barr