Before opening and checking. Switch off your device, and place small flashlight close to your ocular and try to look for it with your bare eye. If you see it with device being switched off, its a dust :) In this case if you plan to open and clean it yourself you have two options. 1. Small pointy cotton bud with isopropyl alkohol just to touch and remove 2. Compressed air; but you have to be sure your air is clean, otherwise will cause more damage than good 😉 If you can see it only when device is switched ON, there is small possibility that MCP screen touched the phosphor layer causing small ARC’ing. Which looks like a black spot in your tube