I'm not sure why you would need to remove the top panel. Is the stain, whatever it is, running inside the bezel or something?
If it salt, I would think it would be easy to clean. I would start with the Apple-recommended damp cloth. I have never had anything on my case that I couldn't clean with either air or a damp cloth. If that didn't do it, I would probably add 1 or 2 drops of mild dish detergent (like Dawn) to the water and try it again. Go slowly, don't rub to hard, and start out with a small, out of the way area.
Whatever you do, it is pretty clear that Apple thinks you should stay away from solvents and the like, so products like Pinesol or Formula 409 are verbotten.
If this is an ongoing problem, you might want to look into
one of these.