Gorilla Super Glue is water resistant, but not waterproof and it should breakdown over time using hot soapy water, but numerous applications may be needed.  You can us a towel dampened with acetone or isopropyl alcohol and wipe the surface, allowing the cloth to remain on the surface for approximately 30 seconds after wiping.  Then, using a moderate pressure, rub the surface.  The adhesive should begin to soften, but many applications may be necessary.  Personally, I would stay with the soap and water because I am not sure what these chemicals may do to your decal!

I hope this helps!