'''++@gallina++'''
In all reality, it should not matter what it is but that you need to get rid of it. Check for a fungal LCD on here [post|356731] Whatever damage it is you can always try and disassemble the actual panel. Most of the damage is most likely on the diffuser sheets. The panel on this iMac can still be disassembled to get to it. You could try and clean (use bleach for the system) those sheets or replace them. Just be careful when you do take it apart that you do not mix up or flip the sheets. The proper order and direction needs to be retained. If nothing else, you can also replace the display relatively inexpensive and fairly easy.