It would be dust on the sensor, it happens on any camera with interchangeable lenses. First use a rocket blower to try to dislodge any loose dust. If that doesn't work then your options are to send it to Fuji to clean, take it to a shop to clean it, or learn to clean it yourself but keep in mind the sensor can be a sensitive part. Dust on the sensor is inevitable no matter what you do, however keeping the rear element of your lens, the lens mount, the camera mount, the rear lens cap, and the camera body cap all as clean as possible will help keep dust off the sensor. I also suggest changing lenses with the camera facing down and off.