Is it on the top of the water? Blanketweed. Koi will often eat
it...a benefit.

It cannot compete well with vascular plants. How are your other
plants doing. We get blanketweed on our berm ponds (that have no koi)
before the vasculars get going strongly (hyacinth, water celery,
parrots feather, etc). Once the others get going, the blanketweed
'rots' away. You can pull it from the surface with a brush or your
hands. That pulls the nutrients from the pond, a definite plus.

You will also benefit from plants that cover your pond surface such as
lilies. They ewffectively shade the water.

You may also get green water if you have too much nutrient in the
water. Same cure.

Have you a veggie filter (smaller, shallow pond loaded with floating
plants to filter the water and grab the nutrients)? Rule of thumb:
Veggie filter with 10+% the surface of the main pond. More is
better. Draw water from the lowest point of your main pond to the
veggie filter. Good circulation and good muck collection. Our berm
ponds collect the muck from the main pond. We drain them once a
year...they have inches of muck. We never have to clean the main
pond.

Good luck.