Polyp eating is a common behavior with these guys. I'm surprised by the number of people here who say they've never heard of this. Search the SPS forum on RC and you'll see that the majority of SPS keepers refuse to keep clown gobies in their SPS tanks for this very reason.
There are a few people with rather large tanks that put up with the polyp eating behavior because they have many, large, healthy, fast growing colonies in large tanks that grow faster than the gobies can destroy. This is similar to some SPS keepers housing species of angels and file fish that are known to eat SPS polyps - these hobbyists put up with some coral eating to keep a diverse array of fish they enjoy.
Personally I wouldn't chance it in a small tank.
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SPS Dedicated 24x24x20 Trimless Tank | 20 g Sump | Bubbble King Mini 160 Protein Skimmer w/ Avast Swabbie | NP Biopellets in TLF Phosban Reactor | ATI Sunpower 6 x 24W T5HO Fixture | EcoTech Vortech MP20 | Modified Tunze Nanostream 6025 | Eheim 1260 Return Pump | GHL Profilux Standalone Doser dosing B-Ionic | Steel Frame Epoxy Coated Stand with Maple Panels embedded with Neodymium Magnets
"Mens sana in corpore sano"
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