Hi bro,
Weishun is right. Plate coral, Heliofungia sp. are solitary corals that exists on the seabeds.
The juvenile plate will begins its life stuck to a liverock and will break itself from the LR and end up on the substrates. All plate corals like to move about thus never place them high on LR on any surface where it can topple over and trap in corners.
When the LFS told you that they dun like sands they are 1/2 right only. Although plate corals likes to be on the subtrates, they do not like to be covered by sands. Thus if you have sand sifter gobies that constantly drop sands on your plates corals, this will cuz them to be stressed as well.
I do not know what are you water params and what are the stocking conditions as such unable to advise you properly as to what is the actual cause of your plate being stressed.
However, it would be wise for you to place them on the substrates without any disturbance in hope that they will recover soon enough.
As for your dead rocks, I dun think those are natural dead rocks from the sea (I may be wrong). Always use rocks with marine origin as rocks from the land may contain high concentration of phophates and silicates or even copper that can be deterimental to you tank in a long run.