I haven't done deep research into this, but apparently, Musk isn't in the US legally. I've heard multiple times that he came in under a J-1 student visa back in the day, but went to work instead without updating that visa. Given his current citizenship status, company ownership, etc no one would reasonably pursue that case, but a particularly annoyed and vindictive person could, if true, say that he gained his citizenship under false pretenses and deport him.