Spray Bannix on it as often as you can! It seriously works miracles for my horses :)
You can try rubbing bag balm on your horse's heals or use M-T-G. Works for me every year. Start with the M-T-G and once it's started to improve use the bag balm.
It's normally called cracked heel or scratches. There are many different ointments you can put on there feet to help prevent it. If you notice the scabs or sores are not going away. Contact your vet and they can give you a specially made ointment. Maybe try using electric fencing to keep your horse out of the mud. I'm not sure if that helped or not ;)