shouldn't be on the rocks, they are sand dwellers and the rock will cut their flesh, its bleaching and if you don't get it into shade it will be gone pretty quick.
These are some common reasons for bleaching, I doubt yours was lighting, more likely tank parameters or temp
increased (most commonly), or reduced water temperatures (bag transfer)
increased solar irradiance (photosynthetically active radiation and ultraviolet band light)
changes in water chemistry (in particular acidification) You really need to know your's and the lfs's parameters
increased sedimentation (due to silt runoff)
bacterial infections
changes in salinity
herbicides
low tide and exposure
