mold in outdoor run

[saildog]

I have an outdoor run, but mostly the chickens free range. The feed and water is all kept in the outdoor run though., which is roofed. It is dirt, and then I put coastal hay down. Anyhow we have had a TON of rain and I am seeing MOLD in patches on the ground. I raked the hay out and put bleach down but it keeps coming back. Any ideas , should I be using sand or a different substrate, or just leave it dirt and not add the hay ? I have 25 chickens and it is a 15 by 15 run, they have a 15 by 15 house, but as I said they free range all daylight hours and go in the run as they please for the food and water that I put there.
The mold is not on the food, just the ground.