It's the ocular power to visually differentiate Hashirama's cloning. Madara said "only my ocular power could see the difference...." he had MS back then when Hashi started using Clones.
But in a way Kishimoto really did a nerf in Madara's MS abilities, he gave one silly ability which is not even an offensive power.
No..ems grants that ability. Sasuke noticed the difference between kabutos clones while itachi with regular ms didnt the real between the fake
Maybe that meteor justu?
Its not a special ability?