Providing you did wait 24 to 48 hrs before applying your decals, there are a couple of reasons why you have stains in the areas you applied decals. First, if you used a setting solution (MicroSol, MicrSet etc) they can leave a stain. Second, the glue on the back of the decal will often leave a stain (the amount of staining will vary from mfg to mfg). The cure is relatively simple, simply let the decal get completely dry (I wait 48 hrs) and gently wash the area with warm soapy water, rinse and let dry.
And BTW, Welcome to the Forum