This reminded me of a problem I had with my monitor last year - an actual bug had crawled inside the screen and died. I assumed it was a cluster of dead pixels and was planning to replace it. Eventually I realized what it was and learned you can use an electric toothbrush to migrate the dead bug to the edge. No luck in my case despite my best efforts. But later when I returned from a month-long trip I noticed the bug was gone and my monitor was once again clear.