sometimes there can be a discoloration around a decal,,,,,,,usually looks like a pool of saltwater run down from the decal and evaporated there, when I still lived on the farm it could look like dried rust sometimes (well water with very old pipes)
I used to have this happen until I started using distilled water, warmed in the microwave, to apply decals
Now, I see a small amount of it once in a great while
Clean up is still the same, if you are painting with enamels, use a little Windex on a cotton swab to clean the area after it is dry
if you are using Acrylics, then use a gentle mixture of dish liquid and distilled water,,,,,,with a second attempt with distilled water only to make sure you leave no soap behind