If you've seen him before, multiple times, is it the same rock? Remove the rock and tada, he's gone. Otherwise just wait for him to surface, then remove whichever rock he ducks into.
Just take whichever rock he goes into out...swap or sell it at the LFS, or quarantine it in an empty tank until the worm decides to go into a trap or leave the rock for some reason.
live rock is fine out of water for around 2 hrs so all i do if i have a large fireworm is remove rock rest over the tp of a bowl of freshwater the worm will soon come out and drop into the fresh water, instant death they will always come out after about 30 minutes looking for water then place rock back in tank no harm done.
I was told to get a bucket, make the water salty, like 1.040, then add the rock in there. He said it would stun the worm, and he would die because it was too salty.
Yea I'm having the same problem only I think mine's worste. I have those little critters everywhere from my LR to under and within my substrate. I even have 3 or 4 (4-inchers I believe) stuck inside my filter itself on the first and second chamber. I believe my only solution to my filter problem is to use a turkey baster, but I also don't want to ruin any anaerobic bacteria already established in there...but I guess I have to do it. For my LR and sand problem however I'm still trying to solve that. I even tried using tweezers but they're too fast...once they feel threatened they hide back. I'll let you know if I do succeed on something. Good luck with your critter problem and keep us posted.