So many Zoro eye stuff....I'm most cases I would say that its a cut eye.
Other than that, I made a theory saying all cursed swords has a soul/spirit trapped inside like souls are trapped in DFs. The spirit of Sandai Kitetsu is pretty demonic and wouldn't allow CoA on it. In order to make an agreement, Zoro gave up his eye to the Sandai spirit to get it to cooperate(Zoro is a guy who tried cutting off his own legs just to fight Mr 3). In recent chapters when Zoro had Sandai coated in CoA, it gave off a weird demonic aura and Fujitora said "I don't know who or what you are.."
Either that or just a cut eye.