is it new to you? if not, did you recently switch to LED?
I found that when I first switched from t5 to LED, my euphyllia lost a lot of color (becoming very transparent) but always stayed fully open and looking happy. within 4-6 weeks they regained full color and never looked happier.. I'm not the first to experience this, I've heard it from several people and it's simply how the coral adapts to the change in lighting. it can take a while for the color to bounce back.
I'd keep it well fed and don't blast it with light, it should recover easy enough..