I have never directly experienced what would happen if a clam's mantle touched a zoanthid, but I would try to avoid having that occur.
It might get stung, or it might just be irritated, but it certainly can't be good for the clam and not something that *I* would risk on a lovely maxima.
I've had them touch without issue. But I still wouldn't recommend it as zoos are known toxin factories and problems might develop from chronic exposure.
