I've not heard of shading a pond helping with grass control. The shade would help with algae control and it looks like that's a problem.* If the grass is not native and a weed problem, certain fish can help to control that.** I'm wondering what chemicals you used and if you have an id on the grass.*
I would suggest you contact your local extension service for help.* They might even refer you to your local Department of Natural Resources.
I'd love to know what you find out.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.