You could just turn it into a sort of "weakens and corrode" type of thing. Mix up poison and diseases. Highly effectively against the grimm since they're more or less "Flesh and Blood" and lack an aura, but those with an aura is more resistant to it thanks to it's protective effects. Functionally, it makes the grimm more sluggish, their attacks weaker, and their defenses lower.

One of his weaknesses could be the friendly-fire potential of his disease cloud since, while most hunters are resistant against it, they could still be affected if caught in the range. An additional weakness could be that it's pretty much useless against machines, since if you're still working off the disease angle, you can't really give robots a flesh-eating sickness.

I'm guessing the general theme of his semblance is "Plague".