Grab the base firmly and place a sharp knife on the clam's base and gently push it to the left, scraping the clam off.
If the zoas are growing on the clam's shell you would have to simply cut off the zoas from it and pull the clam off.
Please be careful with palytoxin!!!
Make sure you've got protection while doing that (goggles, gloves and keep your mouth shut.
Good luck,
Grandis.