Without pictures, it is difficult to make an accurate comment, but if you are talking about what I think you are:
That is not whiteline on the coronary band. Whiteline medicine is caustic as it's purpose is to kill the fungus that gets down in the wall separation, so putting it on the coronary band would not be the thing to do
Whiteline, a/k/a seedy toe, is only found where there is a separatinn of the hoof wall and sole.
I am not good at explaining things, so hopefully someone will come and and say this in a better way:
That "white line" you see is what I would call the horse's cuticle. It is normal for it to grow down and shed.
With all the rain you are having, it is also normal for that "cuticle line" to appear a lot more prominant. It is not anything to worry about.
I hope this helps