I've always thought it to be a dirty cloud layer on the Endor Moon

It does seem a bit odd though that the Rebel Fleet take the fight to the Imperial Fleet (to avoid the Death Star) only for The Executor (and ships of both fleets if that gray blur is the Death Star surface) to be so right on top of the station the second the shield is down so that the Imperial flagship can plop to it's doom.

I suspect that storyboarded plan to have the capital ships attack the Death Star was still in there when they filmed these elements but like the extended raid on the bunker we only have a few nonsensical fragments left.

ROTJ is such a mess.

It would be nice to get a bit more variety in what we see of The Executor (not just the bridge shots, the few times we see the ship in any state other than the traditional ESB side on view we lose the city lights and it becomes a dull shot of black boxes).

That would possibly mean making a number of models of different sizes and designing mattes.

A few people have given it a go but I doubt if the would lend their pride and joys for a fan edit (never know though).