mukuro is a top notch illusionist with a flair for dramatics -- so illidan will be able to see him, but he will not have a distinct "form", so to speak. it'll be like talking to a mist. he will also not be able to see how powerful mukuro is, because be hides it under a powerful illusion. if that makes sense? i hope this makes sens lol, lemme know if you want to hash things out.