Oh I see! I think I agree with you about drawing it separately, though I would add some kind of reference to it, perhaps in the legend? I don't know.

Just as a side note I also have a shadow cast on my fictional world, but in my case its the permanent shadow of a twin world. I never tried to map both the worlds on one map. Instead I mapped the first world (Ethran) showing the fall of the shadow of its sister world (Errispa) on the surface, just as you have done with your floating continent, but I also added a reference to it in the words I added to the map in the top right corner - as if written by the cartographer.

Ethran words.jpg

Just an idea