Hi,
There is a couple ways to solve the problem.
1. Remove the rock with the clam and dip the rock in a 50/50 mix of 3% hydrogen peroxide and tank water. This may not be a viable option depending on size of rock and position of clam on rock or other corals on the rock. For the area around the clam shell you can blast it with freshwater to kill any flatworms hiding by the clam. A small syringe would be good for this. Drain the freshwater away from the clam though. A healthy clam will not have any problems being out of the water for 15 to 20 minutes.
2. To cut the byssal threads you need a sharp x-acto hobby knife. They are small and thin to get around the shell. First step is to get the byssal mantle to retract by gentle touching it. Once it has fully retracted you can work the clam back and forth and cut byssal threads everytime. You can laterally(90° to shell opening) tilt(up to about 25°) the clam slightly sideways to get at the threads without causing it damage. Do NOT pull the clam straight up.
3. As Tony mentioned clams, and also anemones, will detach from rocks if they are in an area they don't like. When clams do they leave the byssal thread mass behind, it looks kind of like a whitish 1/4 of a walnut.
Cheers,
Tim
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