What is this? It's under the surface. It's like brownish yellowish in color.

And should I make an effort to remove from the bottom of the pond?

I suggest you post a picture of it taken out of the water, against a white background.

I agree with Theo. No way to ID it from the picture. Guesses don't count when you are trying to treat it.

I got some grass carp yesterday. Do you think they will eat this?

I also took a picture of it outside the water. Here's what it looks like.

I did a reverse google search and ONE of the results was for a hydrilla. Does that sound right?

Ray:

Some tips for taking pictures of weeds that you want are identified.
Take a white sheet of printer paper, put it on the flat ground, table, etc. any flat surface.
Take a glass baking dish that is about the size of the paper, put it on top of the white paper
Pour about 1"-2" of water in the dish
Lay a single strand of the plant in the water so it will expand naturally.
THEN take a good close up picture of the plant in the dish.

You can look here too,
https://aquaplant.tamu.edu/plant-identification/

I can clearly identify dandelions and red clover.

I went to the suggest site and I think it's widgeon grass. and carp eat that.

The 2 pictures that you posted are not the same plant.

Yeah, different plants. One of them looks like widgeon grass though.