Mud fever is horrible and can be very painful so it is much better to try to prevent it! You are right not to be washing your horses legs, as it is not caused by mud, but by constant wetting. The skin chaps and the bacteria gets in, causing hairloss and raw looking oozing scabs to form. The bacteria breeds under the scabs, which the spread up the legs and can become more infected, possibly needing antibiotics from the vet.
Check your horse daily for any signs of scabs around the heels, pastern & fetlock area. If he's ok apply a barrier cream to clean dry legs and then turn him out as usual.
If you have scabs or scaly, sore looking areas, try to get him into a barn or onto dry ground. Let the mud dry and brush it off.
You must remove ALL scabs, clip his legs if he is really hairy then clean the skin with an anitbacterial wash, ( a diluted iodene solution or hibiscrub is ideal ) The skin is now sterile and clean so apply a soothing cream to any raw areas as neccesary and he should now heal up pretty quick. Try to keep him either inside or out of puddles and mud if you can untill new scabs are forming. If you have no options but to put him out then apply a really good barrier cream. Make sure it is covering the whole area, and smooth down the hair and any feathers to form a complete water proof layer.
Inspect him daily untill the ground dries up, and if the new scabs show any sign of infection you will need to get an anti biotic cream from the vet.
This winter has been horrendous for mud fever due to the wet, mild weather. ( In the UK ) My lot have so far wintered out, happily, with plenty of hay on the dryest part of the field. But, two weeks ago we got the scabby heels in the native pony! My TBx and the Arabx as well as the Welsh cob have been fine, but the greedy Highland pony stands at the gate all day ( in a big muddy puddle ) looking for food! Guess who got the mud fever? Bald Highland? Shaved legs? Not an option as show season will soon be upon us!
I keep an eye to the weather forecast and bring him in when it's wet.
One really good tip: if the mud fever scabs are too hard to remove, apply baby oil generously then wrap the legs in cling film over night while the horse is "in"
By morning the scabs will be soft enough to pick off easily! ( Gross isn't it? ) then scrub & carry on as usual!