I have battled this algae a few times over the past 25 years. Black beard or black brush (there are two quite visually different forms, you have one of them on the Anubias) is caused as someone said by an imbalance of light/nutrients. Once you establish the balance needed for the plants in the tank, you will not see this algae increasing. And the aim is always to restore the imbalance and prevent the algae increase; there is no harm leaving what is there in there, except eventually you might remove the leaves it is worst on if the plant is growing new leaves that remain free of it. And tyhat is exactly what will occur when the balance is stable.
Light has two different aspects: intensity and duration. These are not interchangeable, so if the light is say too bright, reducing the duration will have minimal effect (sometimes none, depending), and similarly if the light is not intense enough, increasing the duration will certainly not compensate. Intensity of light is what drives photosynthesis, so it has to be adequate for the plant species. Algae is always able to take advantage of any excess/insufficiency be it light or nutrients, which higher plants cannot do.
Anubias is a slow-growing plant, and that means it requires less intense light and fewer nutrients. It is very common for this algae to appear and spread over Anubias (Java Fern is another) and it usually means the light is too bright. Floating plants to shade the Anubias can make a big difference.
Light is the usual factor here, but I have had this algae appear with too much plant fertilizer, too little plant fertilizer (for the plants), too much light, and too little light. I have seen the algae appear solely because the tube was wearing out and I forgot to replace it after 12 months. I have seen this algae increase due to the longer and brighter day in the summer (fully darkening the windows in the fish room solved this).