Hard to tell from those pictures but if your state is anything like Wisconsin, Eurasian Watermilfoil is a strong candidate.
If attempting to remove that stuff manually, be advised the tiny fragments left behind, will start new plants.
Not that removing manually is bad, it lowers the nutrient load. Where as just killing it, would let it fall to the bottom and turn into fertilizer, for next years batch.
The good news, your pond is small enough to manage without breaking the bank. Pond dye will help reduce sunlight from the deeper plants and would be a good now and as a followup. Spot treating with a granular herbicide would help for sure, but if it's in your pond at all it will continue to spread from what you missed. Floridone will wipe it out completely, along with any good plants.
If it were me, and it was not long ago, I would go Floridone and Pond Dye to eradicate that nasty stuff.
