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I think I had (and used to have a lot more) of the same stuff. It is a dark red (burgany) and grows in thick patches. Some areas have thicker branches than others (in low flow area it seems). I have had to manually attack it only and it is slowly going away.

All I can say, is don't wait around for it to dissapear. Get rid of it manually as fast as you can. It will spread like a weed otherwise, mine did....