Each species of coralline algae has a very specific set of requirements with very little wiggle room. The factor we encounter most often is lighting. It's not uncommon for someone to replace old bulbs and be shocked that half of their coralline dies within days, only to be replaced by a different color. What has happened there is that the kind that died liked lower light, and when the bulbs were replaced, the intensity increased beyond their tolerance.
In your case, I would guess it is the SG
or temperature. But it may bounce back once things are stable. I've had patches start to turn white, then suddenly recover.