Pig oil and sulphur!!
It's amazing, I have a cob who's feathers are prone to mud rash which breaks her feathers off- I've been using pig oil and sulphur for a year or so and it's Helped so much
Feed copper to prevent. They've linked copper deficiency to mud rash.
Washing them, applying hibatane and even gently rubbing it away will probably help
My vet made a specific mud rash cream and suggested to wash with hibi scrub before applying. Ive found that mother bee stuff works well!
Wash them down with hibiscrub and apply either 'stinky stuff', or the main ingredient is Neem Oil which you can buy on eBay.