I had the same thing happen to my GSP after being green forover a year in my tank. Over the course of a month they went to a purpleish brown and my wife even commented I should toss them and start over. Nothing had really changed equipment wise but I had been lazy about water changes and testing. I did some testing and phosphates were a bit high, nitrate was at 0 but my alkalinity was at 6.5. Calcium was somewhere around 400, maybe 390.
I performed a 50g water change(RSCP) on 120g water volume system, raised the alk up to about 8.5 over the course of a couple days. I also started feeding heavier to try to bring my nitrates up a bit as well as restarted my RS Reef Energy A&B dosing. GSP started showing some green within a week and were fully recovered inside of 3 weeks.