I dug into the .zip file in the Mac Library.
The file not found is hr-boiling-green-patch.png, and in fact that file is not in the chemical-plant folder.
It does contain hr-boiling-green-patch-mask.png and hr-boiling-window-green-patch.png but not the one that's being asked for.
I can't even see the mod in my mod list anymore. I can download it but it does not appear in the mod list. I deleted the ZIP file, redownloaded it, nothing. I checked the contents and what I can see is that the root folder in the ZIP file is not named the same as the ZIP file itself. The version number is missing. Still, even if I change the folder name accordingly, I cannot see the mod in the list.