Do you smoke or use tobacco in any other way????? With the picture that you have uploaded it seems like a harmless area of melanosis which does not require any treatment and is not troublesome. But if you have any major symptoms in that area or you observe any changes in the colour or size or shape of that area over time, you need to visit an oral pathologist for an evaluation. We would prefer to be on the conservatives mood right now and no intervention is required. Regards.