I think a bunch of us, maybe I'm wrong, attach the corals to their own rock so that when and if you move you can move the coral easier. That is one thing that I do anyway. Once the corals get so big and moving them is really not an option then I would think the coral would have grown over the original rock it was mounted on and spread to other rocks. I think if you are looking to get more and more coral over time placing frags/colonies on thier own rocks will help you resituate the tank for more coral as you get them.

BTW that tank looks like it is a bare-bottom setup, so they may be placed in plastic grates or something similar, but like always I could be wrong.