The reason most of the rarer genicanthus will be more reef safe is because they tend to be found either on reef drop offs or like 100+ meters down. They don’t hang near rocks and are more open water fish but coral grows on rock and can’t grow on sand (Unless it’s Xenia in which that can go from in the sand bed to a rock structure). The deeper down you go then the less coral growth there is too so down there it’s probably just NPS corals such as chilli, sun coral, black suns, NPS trees ect.. I have my melanospilos in a tank with a chilli, frammer, favia, Ricordea Yuma, discasoma soo (Blue mushrooms), Monti caps and have kept him with a Staghorn and Monti digis. He has just stayed up close to the surface and only uses the rock when scared and even then he just hides round the back.