My reds did the same a few times. They appear to enjoy scavenging for food. So I hid some mushrooms in the coir/forest floor, and a few little remnants of purple cabbage, and they seem to focus on that instead of the moss or wood chips.
Also, there's a product called Miner-all made by Sticky Tongue Farms. It contains different minerals that tortoises might be lacking in their food...one reason why they eat dirt/substrate.