Does that look like LCD burn-in or is it condensation? It's that way when the iMac is off too. The yellowish appearance is most likely because it was under a yellow light.
Its an easy fix. Go to Walmart or Walgreens and pick up one of those suction cup shower handles. Now take it out of the container and make sure the cups are really clean. Pull up the release levers and place vertically in the middle of the screen. Slowly push down the release levers. Now pull strait out. The glass is held in place by metal posts and magnets. Now you can clean the inside. I use a little windex on a microfiber cloth then I wipe it down with a coffee filter and then blow it off with a can of compressed air. Hope this helps.