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Which coral is he eating? They are detritivores. Are you sure he's just not trying to eat any uneaten food on your coral instead?
Unless they are hungry and there's not enough food in your tank for them, I don't see them trying to eat a coral.
I have had them in my tanks without issues.
How can you be sure it wasn't just cleaning up already dead flesh?
I bet more times than not, it wasn't the hermit that killed the coral. Same thing with emeralds eating dead flesh. They get blamed non-stop for killing what they're just cleaning up.
Never had any issues with hermit crabs coral either, and I doubt it's the crabs fault, but I have definitely learned that in this hobby any creatures can have bad apples in the bunch. Sometimes you'll just get unlucky and get the one with the poor attitude/behavior even though 99% of them would never cause issues. How do the rest of the zoas look? Any indication in the rest of the colony of declining health? I've often found missing zoas to be the work of things like large bristle worms and other hidden invertebrates that are hard to catch in the act so just make sure you've got the right guy.