Some of the senior members here use substrate. Some I would consider very knowledgable and experienced.
Coconut fiber should be pretty safe- it's pretty safe for other lizards.
It doesn't mold, but poop and dead crickets and other organic left on a slightly damp substrate will mold. Keep things picked up regularly.
The benefit of substrate like coconut fiber is humidity.
That said- I'm playing devils advocate with this advice. I don't use substrate myself, but I do use plants in pots that are 17" across full of soil, so I guess I'm basically doing the same thing...