Looks like you have a great start! I would mix up a really strong water/bleach solution and spray the mold down. Let it set for a few minutes then spray it down and scrub it good with a brush. As far as the pea gravels go...skip them, and put 5 or 6 inches of sand on the floor. It drains well and its great for the chickens to dust in. It is also really good at drying the poop out and easy to clean.
Good luck to you....keep us posted on your progress.
Good luck to you....keep us posted on your progress.