Picture sent at the least...
If it was me the vet would
be coming, period.
There is more dead tissue present than you realize looking at it.
For a good heal you need that seriously cleaned, called sedation given and some serious medical look-see and possible cut-back to really healthy clean tissue.
All that that is raised and curling...debridement time.
You could probably use some antibiotics too once you know what kind of bacteria is present at the site.
Your horse needs some sleepy-juice so no one loses their skull when you start to really deep clean and search for something embedded or a cause.
I hate to say it but has it really bled, bled from all areas and sides so the heal is really started?
Just looking at it makes me think you have something going on that needs more than a picture eval.
Hope I'm wrong but gut feeling...call the vet now
, get it seen in person before to much "heal" takes place.