Oh neat, i had that in my 55 mixed predator tank a long time ago, I assumed it was because of being overstocked, even with 50 percent water changes almost daily it NEVER went away.
Eventually it covered all the wood, to be honest, it looked awesome, it would sway in the current like grass in a field.
I left it alone once it grew over 2 inches. Fishes loved it.
But good luck, I could not find a cure for it.