The way I see it, it's overflowing CoA and that CoA smoke is basically a froced substitute up until the wielder of Enma realises he has a whole new capacity with which he can fuel its appetite- CoC. Basically means that Enma is a sword for Conquerors haki users as it is used to late Oden's haki game,: top tier CoC strength and a massive haki tank. Non Conquerors for once don't have strong enough willpower to get a hold of it/restrict it, while they also have a much more limited haki (CoA only) tank, which is why Enma instantly drains them empty.
And the way Zoro might get rid of its restriction is by being able to regulate his output so it gets in tune with Enma's demand, while he's also not overdoing it. Which imo happens when he grows in spirit and when he has greater control of his CoC, both things Enma was used to, cause it grew with Oden and late Oden was a solidified top tier afterall.