It's a reaction in the clear film of the decal caused by aging.. If you apply a decal that has it over a dark surface, it will probably not show up so much. But if you apply it over lighter colored surfaces it does show up.
The only way I know to get rid of it is to expose the decal sheet to direct sun light. Pick a window in your home that is exposed to sunlight every day and tape the decal sheet to it with the decal side facing the sun. You'll need to leave it there for several days, checking it every day. The process can take up to a couple of weeks but will work.
And oh yea, make sure the window you pick doesn't develop condensation on cool/cold days - that will definatly defeat the purpose of the exercise!