Does he always hang out in that GSP or pipe organ coral (not sure which one it is)? That cant be good for him, Im pretty sure both GSP and pipe organ have stinging capabilities, I knwo they can harm corals, not sure about fish.
What are your water perams?
Tempature?
Looking at the picture I dont see anything that leads me to believe the fish is sick, he looks really healthy, so Id take a shot in the dark and say something is different in your water.
I would QT to a clean tank if you can and if not I would do a 25% water change and see if that help. If your temp is too high the oxygen in the water does not break down the same as it does in cooler temps, he may be sufficating. Again, without knowing your water peramiters its really hard to say what may be going on. Can you let us know what you salinity, nitrates, phosphates, ammonia, magnesium, ect. and temperature are?