I think
@BigJohnny might have called this one correctly when he linked to the Reefbuilder's article about the Yellow Tipped Echinata (even though it's not an echinata, probably a tort imo). I've had mine for over a year and it stayed green with brown polyps until I moved it under high PAR (2 inches below surface under ATI Sunpower with Reefbrite LEDs). In it's new position, it developed the same colors as the Reefbuilder's article shows.
Here are pics of it in low light and then about 6-9 months later under high light.
According to Jake Adam's article, this acro was released by ACI Aquaculture. I found mine at a LFS about the same time as the Reefbuilder's article was published.
I suspect your acro is the same one. Did you ever try moving it?