From my experience with brown algae, I have found three things in common:
1 - Newly established tank
2 - Tap water quality
3 - Lighting
1 - I always get brown algae when I set up a new tank. After a few months it goes away; without adding or changing anything in the aquarium. I just set up a new tank a few weeks ago and moved a fake cave from an established tank into my new tank. Within a couple weeks, the cave was covered with a thick layer of brown algae; interesting thing is that cave was in a tank for about two years and had no algae on it.
2 - My tap water also causes it. For example, my sister gave me her 75G tank, fish, and contents last year. It was set up and running for about ten years; no algae and water was clear. I brought along three 5G buckets to move the gravel, plants/ornaments, filter pads & sponges, and fish. A few hours later I had the exact setup running at my house. The only thing different is the water (my public water vs her well water). Within a week, I had brown algae everywhere. BTW - I also have surface oil (protein?) out of the tap.
3 - I set up a new tank and put white egg-crate divider in it. Brown algae started growing on it, but only in a circular pattern near the top by the light fixture. All other areas of the divider had absolutely no brown algae. The light fixture has a low wattage florescent bulb. Two weeks later, I added another light strip (put it next to the original). It has been only a few days; yet I can see the brown algae is starting to fade. So light intensity also affects it.
I am not alone. When Big Al's opened their new store a couple miles from me, their giant shark tank and most all of the fish tank where loaded with brown algae for weeks. Big Al's corporate experts from Canada and their local aquatic specialist couldn't clear up the brown algae any faster than I could. All they did (as far as I could tell) was clean all the tanks daily until it went away.