Nice tank. The hairy stuff looks like Black Brush Algae (BBA). Since you are using the Walstad method, the algae could be a result of the excessive ammonia from the soil leeching into the water column. However, BBA is usually introduced from new plants that have the algae spores on them.
You could remove the leaves that are covered in the BBA or use Excel or H202 treatment on the affected areas.
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