Despite their appearances, pterygiums are not, in fact, tiny alien life forms that have come to make house in your eyeball. They’re not living things at all, really. Just tissue that slowly grows on the cornea. The exact cause is unclear, but scientists believe it’s some combination of genetics and extended exposure to ultraviolet light.
Globally, surfers have some of the highest rates of pterygiums. To the point where the condition is colloquially referred to as “surfer’s eye.” Our own Zach Weisberg had one removed in 2015.