I've run into several cases of "the white eyebrow" as you describe which always follows with a very badly swollen eye.
Not wanting to mess with water change every day, & knowing it wasn't a water issue.
What I have done lately is dropp the affected fish in a 1 gallon tank, Then I use "Mardel" fast dissolving powder form.
I dump the entire pouch intended for a 10 gallon tank in, Leave the fish alone for about 3 - 5 days, (no water change nothing) at what time the eye is all cleared, then I throw them back with their sorority. Has worked every time for me, I found the smaller amounts required for the 1 gallon didn't help, the entire pouch however worked like a charm and I saw no ill effects to the betta's