Well it's pretty hard to call this one, however unless your bulbs are old I don't believe it's light related, well it's defiantly not too much light. It's possible your spectrum is lacking but most likely not the problem.
I would have to say it's more likely water chemistry, your nitrates could be lower and although I don't have much experience with Seachem salt I would bet it's not the best for SPS and may be missing or have low amounts of certain essential elements. Others may have more experience with that salt.
I also had problems in the past with certain corals showing STN and signs of bleaching, at first I thought it was light related but it turned out to be the salt I was using. Switching to a higher quality salt was like day and night.
What's your water change schedule like? and what are you using for equipment?
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