I noticed a while back that my rock is turning green. I couldn't scrape it off so I assumed it was the beginning of coralline but it's been over a month and instead of turning purple it's spreading. The tank is 3 months old, nitrates 5 ppm, phosphate 0.03 ppm, alkalinity 9.3 dKh, and pH is 7.8.
It's not coralline, try dialing back white light. All of my rocks look like that in my softie tank. Unnoticeable under blue light and gives your clean up crew something to graze on, nothing to be worried about.