There are others that can offer better advice than I can. I've only had snakes for a little under a year, now. However, it would help to know what kind of cage you have, and how it's set up - I know with aquariums that have screen tops, I've seen it suggested to cover the top with foil to hold in humidity. Also, that a wet paper towel, wadded up and tucked in their hide, helps to add an extra bit of humidity during a shed. Moss should work the same way, I would think.