He will have a mole shaped like a leaf on his shoulder blade

Abu Basir quotes Imam Muhammad Baqir (as) or Ja'far as-Sadiq (as) (the uncertainty stems from the narrator) that he said: ... There will be a leaf below and to the left of his left shoulder blade, just like the leaf of the myrtle.

(Sheikh Muhammad bin Ibrahim Nomani, al-Ghaybah al-Nomani, p. 253)

Since a mole, referred to here as one whose edges are leaf-shaped, it appears the reference is to one that will be the color of skin, and that it is a mole 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.