Daniel Knop says that clams are immune to the sting of corals. "Corals" can be taken to mean a few things with biologists using the term to mean the true corals or Scleractinia while with the inclusion of fire corals, it may mean anything in the classes Hydrozoa or Anthozoa. I think the reference is to the stony corals and also soft corals (probably zoanthids also). I have heard numerous reports of clams being stung by anemones but I have had no problems with many corals including a Catalaphyllia that was partially covering one of my squamosas for a few weeks.
Personally, I wouldn't risk it, anemones can sting just about anything and its better to be safe than sorry.