Sphagnum moss is infamous for hosting a microscopic and highly toxic mold, Aspergillus. It's a leading cause of aspergillosis pneumonia in snakes and turtles.
I like the ground coconut husk substrates such as Bed a Beast or Eco Earth. They are heat processed in order to dehydrate the fibers and that effectively denatures the proteins in them so they cannot support much if any mold growth.
The Zombie Mama is here!