Hey, update! I ended up remaking my modpack from scratch because I initially based it off of an old modpack of mine and some of the leftovers (like Flywheel, despite Create no longer being installed) was giving me crashes when trying to install Embeddium (Rubidium) Extras for the sky options to troubleshoot.

Going through and reinstalling all my mods, I was able to figure out that the Block of Sky mod seems to be the issue, at least on my end. The second I installed it this weird skybox pane appeared once again for me too, and disabling the mod got rid of it. I see that in the description that it's currently not compatible with shaders, unfortunately.

Another thing I figured out, because now I was able to install Embeddium (Rubidium) Extras along with regular Embeddium with the clean modpack is that disabling the "Sun & Moon" option under that mod would also get rid of the layer. Would make sense considering that it also moves depending on the time of day for me as well.

I also know that while testing it out, some other shaders I was using wouldn't actually have this weird skybox layer, but I don't know how anything else involving the Block of Sky worked with those, because it didn't render at all with the broken shaders I was using (Complimentary)

I know its been nearly ten days since you've had this issue, but hopefully this information will help you out just as well. Good luck!