Monitor black spot

[Catman007]

Guys I have a problem with my monitor. I got myself an used HP Compaq LA2405wg monitor for a awesome price. And I tested it in the store and all but I failed to notice this stain or something. Don't know how to call it so I uploaded 2 pics for you to try to help me out. Good thing is that its not that easy to spot I guess. Like when I play video games I cant see it. (that's why I didn't return the monitor cause its not really something that bothers me a lot). But if I can fix it I would gladly do it. I tried to clean it cause I though its on the outside. Well that still could be the case for all I know cause maybe I haven't used good cleaning product. Hope you can help me with this. Pictures are bad because my mobile phone camera is lame.

[LUUD18]

Return it to the store. Where do you live? In Europe stores have a policy that you can always return faulty products within 2 years.

[Catman007]

Well, I can return it even if its used. But honestly its not bothering me and I got it for like 30 dollars. On the other hand if its something that can be fixed somehow I would like to try few things. If not I am keeping it anyway. Its not visible during gaming and its transparent so its not blacking my view. How should I put it : its just there. So if anyone knows some method of cleaning or running some tests or had a problem like this pls tell me what this thing is.