Why does screen fade to blue?

My Mac keeps suddenly fading to a pale blue screen when I am in Safari or any other program - in the middle of the screen it names the program I am in, and I have to click to get back to it. Also it will suddenly go blue and the open windows separate instead of being behind each other. I don't know why this happens and it is a nuisance - it just started doing this a few weeks ago. What is going on? Any ideas?