What's on my plaster molds?

My partner and I got lucky and found ourselves getting a bunch of free plaster molds that were sitting in a storage unit. So we were wiping them off because of how dusty they were and we checked on them today and some are still a bit damp and having this fluff coming off of them. I imagine it's caused by the dampness of the molds and that they need to properly dry, but what is the fluff?