See another plastic surgeon to have the stitch removed before infection occurs.
You have a common complication of otoplasty, and should have another surgeon look at the suture, trim it back, or remove it before infection occurs. It may not matter as far as the end result after this long.
It is possible that this stich would be best removed. If it becomes exposed it could become a source of infection of the ear. Depending on how many other sutures were placed, and where AND depending on how strong any scar tissue is within the operated area, you may or may not need to replace that stich at a later date.
It can be removed under local anesthesia.
Suture visible behind ear 6 months after otoplasty
If you are not able to see your surgeon, I would recommend seeing a plastic surgeon or facial plastic surgeon who performs otoplasty. You need to be evaluated to determine the proper treatment.