Computer monitor has a "splotch" on the screen

I've been using an old CRT monitor. When my aunt got a new system, she gave me her HP w1858 flat panel. It works fine, but I noticed it has about a 1/2 inch long, narrow spot on the lower, right side. I tired connecting the monitor to my old system (Windows 98) and the spot was still there. It almost looks like some light brown liquid ran down the screen. I cleaned the screen surface, but it's still there and it doesn't feel like there is anything on the exterior surface. I tried to take a screen shot, but the blotch doesn't copy, so I photographed it (see attached photo with the splotch circled in red near thr mouse cursor). I think it might be something on the inside of the screen. With my OCD, I find it distracting, but probably not worth trying to get rid of it, unless there was an easy way to do it.

I don't feel anything there. How could something get inside the screen?

Download monitor test cards, and see if they will alter properties of the mark, once projected on the screen.