My guess is too much light. My Oompa Loompas would lose red when in too much light. Once they got shaded the red started to come back.
I like to keep all of my zoas in about 80-100PAR. That does well for most. Although some like blue lagoons and Illuminati want more. But if you ask the guys at Cherry Corals, they keep all of their zoas in about 50 PAR.
I found that at lower PAR that colors of zoas are darker and richer with more contrast and polyps get bigger. At higher PAR the color can get more intense - but then they can look faded and polyps can shrink.