I heard that mongolian spots disappear after puberty. What if it doesn't disappear?

No complications. Mongolian spots are bluish markings on the skin, normally on the lower back, often noted in the newborn period. Mongolian spots are common among darker skinned persons, such as those who are of asian, east indian, and african descent. They can be mistaken for bruises. They often fade with time and are usually gone by late the teen years. They cause no complications and do not need treatment.