Looks like you had trauma to the tooth. If you are continuing to traumatize it like a tic (chewing on nails, etc) it may get worse. If you are grinding and occluding (biting) on that tooth more, may need to adjust it so it won't torque the tooth too much.
Treatment options are to do nothing which is cheaper, less painful and traumatic, and the defect is not very noticeable. Gum graft may be an option which is expensive and can be very painful and traumatic. However, it benefits by reinforcing stronger keratinized tissue.