What's eating my hammer frag?

[schaNYC]

What could be eating thru my hammer? The polyps are untouched, just the trunk damaged. I have a strawberry crab and red leg hermit. Assorted snails. Pair of clowns and a pair of chromis.

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[cody0709]

The crab

 

[gpscuba]

God dam that's not cool at all