That looks like diatoms to me.
I would move the birdsnest temporarily to higher flow.
Diatom algae require silicates to grow. Are you sing RO water? Silicate may be present in your tap water and may be the source of the problem. If your water supply has moderate dissolved silicates (3-4ppm), you will likely see them in the lower-light areas of the tank. Using reverse osmosis instead of tap water can be a good way to fix the problem. Another way is to remove silicates with phosphate removers such as Seachem’s PhosGuard, Phosban, ect.
A good maintenance routine and regular water changes will help lot. In established tank, if a brown algae bloom is followed by cyano, your problem is dissolved organics compounds in the water and a good skimmer will help remove them. If you maintain good water quality, in time (weeks to a few months) the diatoms are likely to die back or even disappear. Diatoms don’t have a firm attachment to the rock work so they can be cleaned off easily through siphoning. A good way to control their growth is to remove as mush as you can during water changes.