I had this problem and it was a pain. I had taken some terrible advice and had my quail on sand for the flooring and this was the result. The poo/food balls were practically hard as cement. To get the poo/food balls off the feet I filled a shallow basin with warm water and one-by-one held the quail so their feet were hanging in the water and their chest/body supported by the palm of my hand. I used a paper towel kind of wrapped around the quail's body to hold the wings closed and prevent them from flapping. I then used my other hand to work at the poo/food balls once they were softened by the warm water and get them off the toes. Luckily none of mine had damage to the feet/toes. Some of the quails didn't like it but some of the others practically fell asleep in my hands. Almost like a spa day for quail. I changed to wire flooring after this incident and haven't had the issue since. If this ever happens again I will probably try the cage with pebbles and warm water, it sounds like a lot less work. 
