Mine did this several times when it was small (frag). It turned out that it was part of the process of creating new heads. Every time a bubble showed up, it would go down a few days later and the head it was in would have split in two (easiest seen at night when the coral polyps are pulled in). When the hammer grew larger, I didn't notice it doing this as much as the new heads seemed to grow from the skeleton, which there wasn't much of as a frag. I wouldn't worry about it.
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