are zoanthids toxic to clam mantles?

[b_newman]

A family of zoanthids have been slowly growing up the side of a clam shell. Now some polyps are at the top and the clam is in bad shape with very little mantle showing. Is the clam being stung or overgrown?

 

[650-IS350]

I would say over grown. Worst case they grow all over the clam not letting it recieve light or properly open.