I need a window treatment for the semi-circle window above a window.

https://www.pinterest.com/explore/arched-window-treatments/?lp=true

They sell curved curtain rods for these. Or you can stain glass it. Or put window film on it. You can use hooks to loop curtains into as well. Pinterest has some ideas, including these:

https://www.pinterest.com/explore/half-circle-window/?lp=true

Window Clings like Stained Glass or Clear frosted /patterned Contact

How about using a cling film product that looks like stained glass?

Start with measurements of the semi-circular space. I would go to a place like Blinds to Go, or a department store home items sections that has curtains.

I wouldn't cover it at all. I love the open feeling it gives. Unless of course it would glare on the TV. There is spray on frosted window look that you could use with or without a stencil.

You could fit a curtain wire on hooks to keep in place and thread net or voile onto it.

If you buy curtain panel that is shorter at sides and longer in middle you could hang that around the arc.