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Looks like diatoms? They feed off of silicates (from source water like tap or certain sand) or nitrate and phosphate. Try bringing down those levels and they will go away. Usually occurs in newly cycled tanks or aquariums with diminishing water quality.
What are you using now? RO/DI is the best..anything else and you are potentially putting in silicates and phosphates into your water, feeding the diatoms.