I found this in my Duncan today. I have also recently lost some Zoa colonies. Could this be the culprit? What is it and how do I get rid of any others?
Amphipod. Beneficial detritivore and scavenger that readily reproduces in captivity and is preyed upon by fish, etc.
There is a very limited amount of anecdotal evidence that suggests they will consume healthy coral, though I would bet that this is not occurring in your case (more likely, they would eat coral that is dying/dead).
I know comparatively little about coral coral care compared to other organisms, though I guess I should ask about how your corals died.