Ducks love water hyacinth!
How big is your pond?
Ducks like plants. If the water isn't moving, it will get nasty. We built a ~1000 gallon raised pond with ~75 gallon filter and pump. It is green, but it doesn't usually smell. I cleaned it about 2 weeks ago after about 5 or 6 months of just skimming the top/bottom.
We did snag some plants from the river on our last kayak trip, but only some of them are still doing ok. The ducks weren't actually interested in eating these ones, which was a nice surprise.
The ducks are going to poop. A lot. And it has nowhere to go. So yes, your pond will get nasty.