Coming to the end of my cycle and noticed this “fuzz” today. Curious as to what it could be. May have to zoom in on the picture. Lights on appears white off appears brownish. Thanks!
Brownish is probably part of the "ugly stage". You'll get a lot of different types of algae initially growing on the rocks, ranging from browns and greens - but it very quickly gets covered by purple coralline algae.
My tanks been up for almost a year and while most of it is purple, you can still find patches of green (etc.) on the underside of some of the rocks.
This is pretty normal for the most part. Good luck with the new tank!