Well firstly, the coco fiber/eco earth is decent at holding humidity, but you must let it dry COMPLETELY before putting it in with your snake. If it is growing mold, it means it is too wet, which means your snake is laying on damp moldy substrate. This could very easily lead to scale rot.

You want your substrate DRY. Even if you are having trouble with humidity, it should never stay wet for more then a day or two.

That being said, cypress mulch is probably the best option to keep the humidity up. I prefer the ease of paper towels though.

Another option would be switching to a tub setup where you will have no humidity issues.