When I have done sphag & bag, I open the bag every day for a few minutes to let air get in and have not had a mold issue.
Other times, I will use a take out container (if it's a small orchid) and use that instead of a bag and just have a small hole punched in it so it can still get humidity while not being closed off all together. I did not encounter mold issues when I did it this way. Good luck!