They can be very aggressive, the small 2-3" piece that we have has at least 4 inch tentacles, and I know someone else who bought part of the same colony as us, and he says that the tentacles he has seen are up to 8-10" long on his piece.
Within a few days of getting mine, a Kenya and an Alveopora that had been fine next to each other for weeks both died overnight. I could have been the GSP that had slid down between them, or maybe the galaxia that was about 5" away from them.