Well I have the same problem with my dogs at times and I use vinegar and dawn soap...and that works great.
How Do I Get Urine Stains Out Of My White Dogs Tail?
There are a few tear stain removers.. Including Eye Envy and Diamond Eyes.... That should do it... Or you can spray it or swab it with hydrogen peroxide a couple times a day, and that will bleach it out over time.
White stains from old soap out of black pants
Wash it with shampoo that might work actually it will work okay bi I I I