In traditional japanese homes you would have paper screens for the walls and doors, you know those shoji screens. Well, a mokumokuren is a haunted shoji screen. If the screen is damaged and is not patched up, countless eyeballs would start to form on the screen and would start staring at you especially at night.
These Mokumokuren are pretty helpless and harmless. You can pluck the eyes out of the screen one by one if you want to and they would still do nothing but stare at you as told in one of the lore. To stop them from regrowing or staying on the screens, you need to patch the shoji screen so that there are no more holes in it.