Well, from what I have observed with one other zoa colony I have, they spread and then break off from the whole so that there is no mat but a bunch of individual polyps. Maybe it's a survival mechanism...if something happens to one polyp, it won't affect the others in the colony by being transferred thru the mat. Just a thought. I have broken the rock and placed it in different areas of the tank, just to see what kind of development I get and to see what keeps the colors the brightest.