One thing you can try is increasing the water flow in your tank. This will help keep the water moving around the coral and may help it to reopen. You can also try gently prodding the coral with a net or toothpick, sometimes this is enough to get it started opening again.

Additionally you can monitor your water and lighting quality. Here's a simple guide:

Calcium: Keep calcium about 380 to 420 for green star polyp corals.
Alkalinity: Keep alkalinity in the range of 425 to 450 ppm for green star polyp corals. 
pH: Keep your pH in the range of 8 to 8.3 for green star polyp corals. 
Temperature and Flow: Green star polyp corals feel great with medium to high water flow and a temperature of about 75 to 80 degrees Fahrenheit. 
Lighting: For green star polyp corals, stick to the lightning from 100 to 180 PAR.

One question that often comes up is how to tell if a GSP is dead. The first thing to look for is whether the polyp is retracting into its skeleton. If the polyp is not retracting, then you can check to see if there is any color remaining in the coral. If there is no color, then the coral is likely dead. Finally, you can check to see if the coral has any translucency. If it does not, then it is likely dead.
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