Laurent - Great picture and information! My GSP came on my original liverock, and they can indeed grow like a weed! But they're very pretty to watch swaying in the current.
Mine came as a small patch on my LR, on the bottom left of the tank, and has gradually covered a good bit of the left side of my rock now. It multiplies by spreading out the mat that the polyps sprout up from. A good choice in the RSM, or any tank, if controlled.
It has overgrown and overtaken some things, but still a nice and colorful addition.