I would use double strength and soak it over night.
Then wash it with laundry detergent and add some more Clorox,
Can`t hurt to try.
Good Luck It might be from bleach or undissolved laundry detergent.
I know from experience that bleach can make pink splotches on blue and gray clothes.
If you have your own washing machine,
you can run a quick rinse cycle to prevent this from happening.
if you share the machine with others,
there may be bleach residue left over from the previous user.