Do not let green star touch any structural rock: they overgrow before you know it and become a plague. They're pretty, but too tough.
I'd recommend euphyllia corals: frogspawn and hammer, plus candy cane. Forgiving and fast growing if happy.
You'll need to start a program of supplementation by some means, plus tests: I use Kent Alk Buffer [8.3 dkh is my reading]; and Kent Turbo Calcium [400-450 is ideal]. Salifert tests. If you can't get the readings to stay 'up', you need the third test---magnesium.
You also need 0 ammonia, nitrate, nitrite and as low phosphate as possible. Phosphate is easiest detected not by test, but by the presence of film algae on your glass.