think of the aperture grille inside the monitor as a weaving loom, threaded very finely, and threaded only in the vertical direction.
The two horizontal lines you can see are the damper wires, which place tension on the vertical wires, stopping them from moving around.
As explained before the vertical line you saw is an out of alignment grille wire. tapping the side of the case caused it to return to its normal position.
Keep an eye on it, if it happens rarely, it's nothing. If it happens regularly it may indicate your tube was not constructed as well as it might have been. Contact the maker in this case.
Greg