Originally Posted by PCoch
........... I'll try a few things next weekend like bleach, steel wool, razor blade, Magic Eraser even, and report back.
Bleach will do nothing for this. Nothing at all. At least it will do no harm.
Steel wool will leave particles of steel imbedded in the gelcoat and they will begin to rust and will be a never ending problem. Never use steel wool on your boat. If you think you need steel wool buy some bronze wool or use a plastic substitute.
Razor blade? Not unless there's something stuck to the surface. Solvent would be a much better choice.
Magic Eraser? That's a very fine abrasive. Wet sanding would be a better choice.
Someone else posted it, you have an area that was protected from UV rays by the old decal. That part is essentially new, the part not covered by the decal has had years of weathering. The only way to get rid of the "ghost" is to bring the surrounding area back to new condition. Wet sanding is the only way to do this other than repainting.