I can't tell from the picture if that discoloration is under the lacquer or on top of it. If it is on top of it, it should rub off with some naptha or wd-40 on a q-tip. (Naptha is zippo lighter fluid).
If it is under the lacquer, you should probably leave it alone unless you want to take it to a repairperson and get the horn disassembled, the lacquer stripped off that spot, the "rust" buffed off, and then the area spot lacquered- which is going to be a bit expensive!
In any case, it isn't going to be affecting the sound of your horn in any measureable way. If it were me, I wouldn't worry about it.