I'm going to agree with the above two. It totally seems like water quality to me. You might not be measuring any nitrates or phosphate but that's probably because all your hair algae and zooxanthallae are taking it up... hence brown corals and hair algae all over the place.
Cubes of frozen seafood are PACKED full of nutrients which your fish eat and then poop back out, or don't eat all together and the algae will take this up. Cut back on your feeding for a bit and try increasing your skimming. It'll take a few weeks of this to get hair algae down and bring colours back.
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