That is a commensal acropora crab, they don't harm the coral, just live within it. A crab wouldn't be doing that kind of damage anyways, there is no way it would expose itself on the top of the coral to snack on polyps at the ends of the branches when it can remain hidden inside the coral and do its damage there.
That damage is either light related or flow related, guaranteed. If you have moved the coral lower or out of direct flow you should see the tissue recession stop immediately and should regrow (or at least start to regrow) over the affected areas within a couple of weeks.
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