Only Kings, princes and princess may be adressed in this way. It is a way to adress a King, princes and princesses.
See https://en.wikipedia.org/wiki/Royal_Highness
It is no title. Geni has fields for titles, names, middlenames and familynames.
HRH can be added to a name, but on Geni that would be in the field "also know as" or "Display Name".
This discussion can be used to discuss the use of HRH on Geni and ask for the removal of the HRH for public profiles that are not a Royal Highness.