Mine has had his mark since the day after I got him and it's 9 monthish later and he still has it... look at it like it's a streak of color in someones hair... simply a cosmedic morph which has no meaning at all but helps you to tell them apart ... I have one with a mark on the tail, one with a white spot on his head and one that is "normal" so I can never confuse who is who LOL