It looks like rain rot to me, too
Keep the brushes cleaned and don't use them on other horses.
If you can, wipe the affected area daily with a 50-50 mix of original Listerine and water. Walmart's brand of Listerine works just as well.
Soak a paper towel and massage it down into his top line.
Try not to throw up if you see black moldy/dotty dandruff come up on the paper towel--------------ask me how I know that one
Annnnd wear rubber gloves, I can also tell how hard it is to wash that crap off your hands, unless you slather yourself with Dawn dish soap
My Arab is really prone to rainrot, anytime of the year. If I massage a wet paper towel into his top line and come up with black (literally black) little dirt specs, he's dealing with rain rot and I start the Listerine/water treatment. I do that every day until the paper towel test show plain old brown dirt or nothing at all
Hope this helps.