Out of curiosity what did the vet have to say about it?
Those little bumps and the scabbing in the earlier pic remind me of dew poisoning. But that is normally a leg issue or seen in areas of white hair depending on the cause. Considering there are so many secondary infections that go with that it could be that you have a staph/bacteria or fungal infection and it just has a similarity in looks to dew poisoning.
Here's a link to treatment options. The Horse | Dew Poisoning/Scratches
Definitely sanitize your fly mask or just buy a new one. Make sure it stays dry when it's on the horse.