How long ago was the IPL treatment which resulted in this white line? If 6 months or less, it might be transitory. If longer, it is probably permament. If that is the case, the melanocytes in that spot have been severly damaged from the treatment. In my experience, the best way to blend that white mark is to treat the surrounding tissue to reduce any pigmentation or redness. This will be an ongoing process though as those melanocytes will more than likely never come back.