After doing some research, it seems that it doesn't take very long for this to kill it's host.
My Molly has looked like this for a little while. I think it might be worse now, but at first I thought the white patch was just a color variation.
Would that change you thoughts on it being Brook? Should I get another Molly that is healthy looking to verify this and treat them both?
I wouldn’t put another molly through this. Just because it hasn’t killed the molly yet doesn’t mean it’s not brook. I would set up a QT and move forward with a fallow period.
Did you acclimate this molly to SW or was it in SW at the LFS?
At the end of the day do you want to put a pair of clowns into a tank that Might have brook? That could be an expensive gamble.