For GSP growing like "weed" is still talking months. Keep its rock not touching any other rock and you can trim the edges every few weeks as you like or less often. It has a clear pinkish growing/advancing edge. If no rock nearby it will advance to sand and will be easy to cut. Green hammer is harmless coral unlike green torch coral. It will get irritated by other corals if touching or if attacked by tentacles like galaxea Corals even if far. For a biocube make sure not to buy any coral with long sweeper tentacles. Eventually in a year or so of there are enough corals, there will be competition for space and a faster growing coral can grow on to another coral. Like GSP growing over zoanthid (has happened to me ) if on the same rock. But it's too early to worry about.