He will have a mole shaped like a leaf on his back

Abu Bashir says: Imam Muhammad Baqir (as) or Jaffar Sadik (as) said: “... THERE IS A LEAF BELOW AND TO THE LEFT OF HIS [Hazrat Mahdi’s (as)] LEFT SHOULDER BLADE, JUST LIKE THE LEAF OF THE MYRTLE.” (Sheikh Muhammad ibn Ibrahim Numani, al-Ghaybah al-Numani, p. 253)

The hadith provides detailed information about the location of this mole on the back of Hazrat Mahdi (as): that it is BETWEEN HIS SHOULDERS; BELOW AND TO THE LEFT OF HIS LEFT SHOULDER. It is also referred to as a LEAF-SHAPED one, so it appears the reference is to one that will be the color of skin, with distinct edges and raised above the level of the skin, thus resembling a leaf in shape and of the color of a faded leaf.