I was going to say that it may be the mh lighting.
It's known that if you place ppe's under direct mh lighting they get white speckles, but looking at the information you list about your tank, I don't think 2x150 mh's will do that,,,or maybe they will.
Try giving them a little shade.
It won't change the color of the already affected polyps, but the new ones that come in should have the original color.