Ick. That’s just gross. I have seen that happen to an iMac that sat in a damp basement for a year or so. I wouldn’t bother cleaning that because [https://epi.publichealth.nc.gov/oee/mold/precautions.html|mold is hazardous].
Here’s a link to a guide on how to replace the screen
[guide|998]
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=== Update (11/24/2018) ===
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If it truly is mold inside your system, your system will require a deep cleaning because as Dan said, any loose spores can re-infect the system and the home. It’s up to you, but I advise that you either deep clean your system or throw it away if it truly is mold.
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On the other hand, if it isn’t mold and rather heat damage as Dan noted replacing the screen will help you out in your case.
Ick. That’s just gross. I have seen that happen to an iMac that sat in a damp basement for a year or so. I wouldn’t bother cleaning that because [https://epi.publichealth.nc.gov/oee/mold/precautions.html|mold is hazardous].
Here’s a link to a replacement screen
https://www.fasttech.ca/imac-20/imac-intel-20-emc-2210-2133-lcd-assembly
Here’s a link to a guide on how to replace the screen
[guide|998]