As far as I know Trachyphyllia should be in the rockwork. It is easy to see wich corals usually live on sand or gravel. They have a plate or disk as skeletom. They cannot have a branched stone skeleton because then they would be slowly buried in the sand. A plate is easier compared to the area so it can "float" on the sand.
Trachys do well in both sand and rock work. By the looks of it, yours should do ok as long as light and flow are adequate.
IME, they do best with lower light 75-100 par, low to moderate flow.
I have several in both locations and they all do well. The only thing to look out for when placing in rocks is that they will fully "stretch out" and their flesh will need some support. I have mine on a pseudo-shelf like rock.
Hope that helps!