That is a lovely case of Scratches, a/k/a Mud Fever
I see you're in NC and I'll bet you have about the same ugly heat/humidity we have in Middle Tennessee and that promotes this kind of stuff.
And it will spread if left unattended. Also comes as no surprise it's on the white leg as the pink skin seems more prone to it than the darker skin pigment under darker hair.
Sooooo, get to cleaning, I'm afraid to say, because it does spread quickly and can end up going clear up the horse's leg.
You've already got the hair clipped back so that's a huge plus as this garbage needs air to dry out.
If you don't have any medicated shampoo, get some betadyne, povidine (any kind of "dyne") and gently wash any fetlock/leg area that you see that stuff. If you see her hair "tufted" anywhere on her legs? That is scratches waiting to get started doing its ugly thing
After washing, pat dry with a paper towel so you can throw it away.
If the scabs soften enough to act like they want to come off by all means take them off. However, if it's an effort to get them off, leave them go a few days because if you force them off the open sore will most likely bleed. That won't hurt the horse except for the flies and bugs the blood will attract.
There are many great potions that many people use to get rid of this stuff. I think much of the success of each depends on each horse. Don't use MTG
. This stuff is not anywhere near healed enough to tolerate MTG.
In this instance, I would use a mix (I should say I AM using) of:
1. Desitin (because it stays on really well and repels water pretty good).
2. Fur-All to fight infection (Corona cream or other antibiotic cream).
3. Equate's brand hemerhoid ointment to take the itch/pain/swelling out.
Others may come in with potions that work for them and it will be something you might have to experiment with.
Those scratches sores will probably take close to ten days to heal if you keep after them once daily. If you can, clean them twice daily until the scabs come off and you start to see pink healing skin. If that's not possible try for once a day (I don't know if you board or your horse is at home
If your horse comes into a stall and happens to stand in his pee spot, I hate saying this, but that pee spot will have to be cleaned daily at least until this heat/humidity subsides.
Naturally my one horse that is really prone to scratches is the one that stands in his pee spot--he is costing a lot of extra shavings and lime lately. I lime the pee spot after cleaning to kill the bacteria. I use dolemite, a/k/a garden lime. Don't buy field lime, it's different.
There are a lot of stall dries on the market, but if they don't say they kill bacteria don't waste your money. A bag of lime is cheap cheap cheap and lasts a long time.
Oh, do not share this horse's grooming equipment with any other horse and be sure to clean all your grooming equipment after each use. If you have Novalsan that is great. I use hot water and Dawn dish soap and that also works well.
Sorry this is so long but I hope it helps, and don't delay the cleaning process as Scratches spreads really fast