'''Wow a very dirty boy!'''
Thats thick dust buildup everywhere the power supply, logic board and drives are just covered! The tape at some point let loose so dust got inside the assembly, so yes what you see is dirt between the LCD panel and the backlight box on the backside.
Before you can do anything you need to give the system a good dusting. Get a can or two of can’ed air, a couple of soft paint brushes and a tooth brush.
Take the system outside and wear a good dust mask and goggles to fully disassemble the system cleaning every nook and crevice.
As far as the display its just easier to replace it! [https://www.thebookyard.com/product.php?cPath=34_122_169&products_id=7205|20” iMac LCD assembly, Apple P/N 661-4671]
Follow these guides as needed:
* [guide|998]
* [guide|1013]
* [guide|1005]
* [guide|1015]
* [guide|1010]
* [guide|1006]
Its a bit of work to bring this dirty boy back, but its worth it!