I'm assuming the first reading you mention (300mVAC ) was between the "radiation screen" and ground.
There is, indeed, some electromagnetic radiation being emitted from the CRT face that is interacting with the screen, generating some, if not all, of the 300mVAC.
The other values you mention I would suggest are "ground loop" values generated by the lack of a properly grounded eqiupment.
And, yes, they can be dangerous.
Now, as far as the radiation from the screen is concerned, determining its composition would require some rather more sophisticated equipment than the meter you have at hand.
That said, as the radition's strength is a ratio of the inverse of the distance squared from the screen, most safety studies have concluded that there is little danger from that radiation if the monitor is used as specified by the manufacturer.
And the screen, properly used, can reduce that value even more, although I don't think that I would bother.