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White on these types of corals is not usually a good sign. They do not grow like other hard corals where you might see the lighter color as the skelatal structure progresses. The flesh on an LPS is usually growing/expanding much faster than the scleractinia.
Could you post a pic? Might just be some fading in color or needs feeding.
To be honest with you I cannot really tell. Is the "white" primarily near the edges of the coral or in various places haphazardly?
Look like just near the edges. In which case I would suspect placement. Could be excessive water flow, too much light or possibley too close to a nearby coral?