The correct look-behind for the em space would be (?<=~m) and what you want to find goes outside of that. (?<=~m).+ will find everything after an em space up to the end of the paragraph. (?<=~m)\S+ will find up to the next white space (so the first word following the em space). (?<=~m)[^~m]+ will find everything following an em space up to the next em space.
It's not really clear to me what you are trying to do here. It's also not clear why you think you need a GREP style rather than a simple nested style.