This is a figure of Medama-oyaji (Eyeball Father) in his familiar teacup. If you expose it to sunlight during the day, it will faintly glow at night. It is a handmade item from Sakaiminato, the birthplace of Shigeru Mizuki (and is officially licensed). I purchased it over 10 years ago, so the eyeball has discolored and turned yellowish. Also, it was originally glued to the teacup, but the glue has come off and it can be removed.