Looks like the little blue Exposé blob hack
*.
You can fix it by reading this Mac OS X Hint:
http://www.macosxhints.com/article.php?story=20031010141631859
* And by "hack" I don't mean to say someone compromised your Mac. I mean "hack" as in, a non-publicly-intended change to an Apple product, in this case, Exposé.
But, I am also not saying that someone didn't do that to your computer, but I would put "compromised machine" towards the bottom of the list.. Hope you understand what I mean.