I've found that Kalk paste will take care of most problems like this.
Pull the clam out of the tank, remove as much GSP as possible, and put the clam back in the tank for a few hours or until it isn't stressed anymore. Mix up some Kalk and water with the consistancy of a batter (something like 4:1 kalk:water will get you in the ballpark). Then pull the clam back out and CAREFULLY spread the kalk paste over the areas of the shell where you suspect there may be GSP tissue remaining - you don't need much. Be sure not to get any of the paste on anything other than the clam shell. It's a good idea to take some slighly damp paper towel and make a ring for the clam to sit on. This will allow the paper towel to collect any paste or kalk laden water from irritating the soft tissue at the base of the clam. Let the paste sit for 2-3 minutes before doing a light rinse with tank water using a baster (holding the clam in the air above a container which the kalk and water will drip into). You don't have to remove all the kalk paste - just the stuff which will come off easily so it won't easily come off the shell when you place the clam back in the tank. Turn off the pumps to your tank to reduce any currents and place the clam back into the tank taking care to not spread any remaining paste on the shell into the tank (if you did the rinse and let the paste dry slightly afterward this shouldn't be a problem). Any remaining Kalk will slowly desolve into the tank water while allowing more contact time to kill the offender that you want to get rid of. Repeat as necessary over the next week if you find you missed areas the first time.
Couple of things to keep in mind:
-Better to do several light applications over a couple days than one heavy one, particularly if you haven't done this type of thing before.
-Keep the kalk away from the soft tissues of the clam! Have a bowl of tank water and a baster available if you do. Should this happen. Dont' panic. Rinse the kalk off quickly and then place the clam back in the tank with some moderate current to minimize the irritation.
-If you have other organisms on the clam shell that you'd like to preserve (tube worms, etc.), only do one half of the shell one day and the other half another day (avoiding getting any kalk on the said organisms of course). This will minimze the stress to these secondary organisms.