where to begin...
It is very possible that moving the rocks around mixed up some of the detritus enough to lower your water quality a bit, thus causing your perc to stress and therefore show signs of ich infection. Is he breathing heavy and eating well? Also, do you see him flashing against the rock work (like scratching himself by swimming quickly along rocks)? These are all signs of ich infection.
There are many opinions as to how to fight ick. The only surefire way to erradicate ich from the entire tank is to remove all the fish from the display into a quarantine tank and treat them with either hyposalinity (lower salt level) or copper. They will have to stay out of the diplay for about 6 weeks or so to allow the rest of the ich that is in the display to complete its lifecycle and die off (it needs fish to live). Unfortunately the stress of moving the fish and treating them with copper (which is toxic to fish also) can sometimes lead to casualties, especially if the infected fish is already very stressed. Also, be aware that there are some supposed "reef safe" ich medications out there, but most have little proven results and can still wreck havoc on your tank. Most reefers here on MR will advise against such medications as well as ever medicating the display tank. (I've tried Kick Ich, and although I had no ill effects, it did nothing for solving my ich problem.)
On the other hand, if only one fish is showing signs of ich, and he is still breathing ok and eating well, you may want to just let the ich take its course and let the fish's immune system fight it off by itself. You can suppliment garlic with your feedings to help boost its immunity, but just be aware that adding garlic will not kill ich, just help your fish fight it off naturally. It will still remain in the tank.
I am sure some other reefers will chime in with there ideas and opinions on the topic. In retrospect, I had the same problem with my royal gramma a while back, where he was the only fish to keep getting infected. I chose the latter method and he's been fine since. On another note, there have been many threads posted about ich. If you do a forum search, you will find a lot of information given by fellow reefers.
Hope this helps. Good luck
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