How long has it been there? It could be bird poop or concrete for all I can tell from that picture. Could you describe it a little more? If you can't wash it off then try using some clay. Maybe some bug/tar/sap remover might work. What have you tried on these mystery marks so far?
IT is some kind of sap and I tried som bug remover, no good. I tried some OOPS...no good. Finally I just cranked up the pressure washer and this took off the white top. I clayed, you name it. I wish I would have had some 3M adhesive remover. I wonder if there is anything my rotary could have done?
Oddly enough, I found that 3M Wax and Adhesive remover that someone mentioned a while back. I used it on Q-tips around door handles and light fixtures for wax and it says it is clearcoat and paint safe. Oh--the "oddly enough" was finding the stuff at Wal*Mart. It is in a metal rectangular bottle. I would use that on a foam applicator and follow with a fine cut polish if need be.