Still think its red cotton algae
What kind of light are you running on the fuge? What spectrum of light is it? How old is the light ? I would try throwing a turbo in the sump to clean it up. If it is no where in your display but in that sump there is a problem in the area, either lighting, flow. Seeing it is in your sump with a powerhead i think the turnover will be sufficient.
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360 gallon sps reef, 180 gal sump, bubble king supermarine 300, 4xmp40Wes, 2 x 6215 tunze waveboxes, 4 ghl mitras
360 Reef Tank