It could be xoolanthae growth spur, cuz of a light change???
I've found that this can be true with anemones, some species of soft corals, and some encrusting species of Montipora (including M. denae and M. verrucosa). But, I doubt this is the reason for the fact that only 175wts of metal halides is present and that the coral is 18" from the waterline.
I would try the "trial-and-error" technique to see which area would best suit the coral, as far as coloration goes. Try placing the coral 5" higher than it is now. See if it regains coloration after a week or two. If not, try moving it 4-5" higher. See if it regains its coloration after a week or two. I would not try placing the coral lower, as it will most likely not receive enough lighting.
Water chemistry could also play a role with this. What's your ammonia, nitrite, nitrate, phosphate, alkalinity, calcium, temperature and salinity (or, as much as you test for).
i found that SPS can change colors just from stress of new/different conditions.
I second that comment. I've added many new Acroporids, and within two weeks most of them turn brown. Fortunatly, as they grow out, they will usually regain their color greater than before.
I personally think placing it higher may make a significant difference.
Graham
