Could it be tree sap, dried bird crap, or tree sap.....if it is one of those try a remover designed for the removal of those contaminates.
If it is just smudged paint, any 3 step waxing product should remove the imperfection. Mother's and Meguiar's both make a 3 step paint care product that should clean it up nicely. Be sure when applying use all 3 steps and do not skip any and use a clean applicator for each of the 3 different products.
Let us know if this helps..... :bigok: