User blog comment:Meruemgungiplayer/A Case Against the emitted Nen Beast./@comment-32818639-20190503144045/@comment-32818639-20190503154531

https://mangarock.com/manga/mrs-serie-111005/chapter/mrs-chapter-100174498

He says: "...that only the user can see...". The princes aren't users of the GSBs and they can not see the beasts even knowing Nen. Also, SM is an ability that can only be seen by the user himself.