Just another spelling for "gray" in pluralis. Grays (sometimes spelled "greys") are supposed to be extraterrestial intelligent non human beings who are captured and held in secret labs deep within Area 51 and perhaps the Cheyenne Mtn base.
In Sweden, the word "grejs" is translated to "a lot of physical things stuffed up all around the office".
"Dude, I saw some grejs last night! Or maybe I had some bad moonshine..."
(Buddy, I saw some extraterrestial, intelligent non human beings last night. Or perhaps I drank too much bad home brewn strong alcohol...)
"Vad det ser ut här! Fan vad grejs det är här inne!"
(Look at that mess! There is too much fucking grejs in here!)