It's most likely bryopsis. The feathery appearance is the tell. In that amount it's not really a concern, but keep your eye on it and maintain good water parameters to prevent it from getting out of control. There aren't many things that will eat bryopsis, so your options are to either water-quality it under control, or use fluconozale (Reef Flux) to eliminate it.
However, in a new tank, I wouldn't bother with fluco. Just keep your water parameters in check. 2-10 ppm of nitrate, 0.05 - 0.08 ppm of phosphate. Stable salinity and good flow. You can also manually remove it if you're confident that you can get it all, but bryopsis is much more stubborn than most algae. If you don't have corals in the tank yet, a blackout might help knock it out.
Most likely, though, you can just keep up with your maintenance and it'll pass. Pretty much all new tanks will go through a variety of algal outbreaks before settling down.