This is a warning to all of you who owns older vintage computers.
By a coincidence I noticed this green stuff on the ball of one of my mouses to my old Atari STs. On a closer examination it turned out it was from the rollers inside the mouse that had started to corrode, probably caused by old dirt and fat on the rollers.
So do check out the mouses. Me myself will clean the mouses and then store them without the ball in place.