Glad to hear that you have 3 peppermint shrimp. They do best in a minimum of groups of three. Often times if yuo just have one they will tend to be timid and shy and not come out as much as if you have a small group.
The green star polyps will likely grow about as fast as xenia will...but hey I know of a great website where you can trade some frags of it when it begins to overgrow your tank!
