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I had this happen in a FW tank and it was an algae bloom I think. I treated it with an algae removing chemical but it was not to be used with invertebrates sadly :/
Make sure to use ro/di water and make sure there isn't excessive nutrients in the water from excess feeding, make a water change of 25% and use the lights as little as you can and keep doing smaller changes until it is fixed. Also make sure that your filters are up to date