What is your watts/gallon and what sort of lights are you running? How old are the bulbs? Are you adding iodine supplement to the tank?
My LFS has a nano reef set up and they have GSP in it. They have lost the green coloration to the. I believe this is due to being in low light situation. They only have 2.6 watts/gallon. Better to have 4 watts/gallon. Their GSP are surviving, but lack the proper color.
I have a 55gal reef as well. I have 8 watts/gallon. GPS are doing great. I have them isolated so they can grow much on other rocks, but still be able to grow up on to the glass. Anyhow, I hope your's improve.
There is also another color similar to GPS. It is commonly called pink star polyps or brown star polyps. However, they are encrusting Gorgonians. The grow on a mat just like GSP.
However, your's looks as if they lost their zoozanthelle.