I think Zhu Mei was saying "We've never seen anything like this rust before, but it must mean that Kanda is dead." So they don't actually know what the rust means, and it isn't to be interpreted as something common or anything like that. (Personally, I think it means impending crystal-type evolution for when Kanda returns.)

Also, Zhu Mei's biases are pretty clearly with the Exorcists vs with the Vatican; if it had been a cardinal saying that, it would've been one thing, but Zhu Mei's interpretation feels to me like "Kanda is probably dead, see, look at this. Now please let Allen go?"

P.S. Cross is definitely alive. I have no doubt! But also no proof.