Pond Boss Forums AQUATIC VEGETATION Controlling unwanted plants 20-year Pond Suddenly Taken Over

I don't think the brown stuff is filamentous algae. The brown stuff looks to have with tiny projections on the strand. It is probably a form of 'higher' vascular plant or it could be an algae called Chara?? .
When I sell an aerator I strongly suggest that the diffuser is lifted and cleaned or scrubbed off EACH year to prevent the diffuser problems that you describe. Neglect causes problems.