Leukonychia (white marks on the nails) is almost always due to trauma or injury. Pushing back the cuticles too hard, for example, will result in white marks appearing on the nail plate surface about two months after it is done. This is due to nail matrix (growth center) damage, but fortunately the damage is only temporary because after the matrix heals, a normal nail plate is produced and the nail with white marks grows out.
There are also instances where nail glue is used in conjunction with wraps or tips may result in white marks as well. Here, simply discontinuing the application will gradually provide a cure as the new nail grows in and replaces the old. — Dr. Scher