Hammerhead Branch Issue?

[Reyphox]

I have just noticed the that near the base of my hammers, the branch shell is fallen off on both branches revealing the "gill" like structure inside. The coral seems happy enough during the day and as you can see has recently split. All parameters are with in proper range. Only thing that could maybe pull this off it my emerald crab but he never leaves the rocks. Any ideas? Anyone seen this?

Sorry about the poor pic but can only see the issues from above so ripples in water throw off the clarity.

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