Is this mold or oyster shells

Well, given your conditions, it's certainly not mold - it looks like the leaves were flooded perhaps with water containing epsom salts or the oyster shells and it's residue from that... I don't think you have anything to worry about. You could take a clean, moistened tissue and try to clean the leaves off to see if the white stuff disappears... Try not to water the leaves themselves unless you're doing a foliar spray... Good luck!