Sudden growth of moles on the neck is generally related to heredity, developmental defects, local skin exposure to external stimuli such as sun exposure, trauma and other reasons.
Acquired nevi are caused by external stimuli to the local skin, such as glucocorticoid ointment on the neck, contact with pesticides and other chemicals, long-term exposure to the sun, local trauma, and changes in hormone levels in the body can cause nevi to appear on the skin.
Most of the pigmented nevi on the neck are benign, and the risk of malignancy is very low, so treatment is usually not necessary. If you have aesthetic needs, you can do laser or electrocautery to remove the pigmented nevus, and you need to go to a regular institution to be operated by a professional physician or technician to deal with it.