Thats really weird... I don't have an answer to this, but would like to find out what causes that as well and is that silicone reef safe after it turns color?
I have a feeling its the silicone or the light over the silicone, if its aquarium safe then there is not much to worry about although it will most likely drive you insane as that tank is a masterpiece.
Hope you get it all sorted out.
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Casual reefer
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