The algae is a form of black brush. Like all problem algae, it is caused by an imbalance of light/nutrients. Filtration has nothing to do with it, in fact I have always seen this on the filter intake/return spray bar where the flow is the strongest. I assume this is because the water flow is bringing more nutrients, but the point is that filtration is not behind this.
Usually it is too much light, either intensity, or duration, or both, or inappropriate spectrum. Higher plants are fussier than algae. But I have also seen is increase from too much planty fertilizers. But it is all balance.
Knowing data on your lighting and fertilizing will help sort out the specifics.