How to remove debris stuck to shadow mask on inside of CRT?

[LukeSimon]

Below is a magnified picture of a Portacolor screen displaying a screen full of horizontal white and black lines. I circled a region of the screen that has some debris stuck to the shadowmask on the inside of the CRT. If you look closely at the black line below the white line with the debris, you can see the cathode ray is bouncing off the debris and bleeding in to the black line, lighting up those phosphors a bit when they should be not illuminated.

Where does this debris come from? From the burnt layers of the cathodes? Is there a safe way to shake this debris off of the screen?