This is a weird growth that some people are born with. It's called a dermoid. It can enlarge over time, as this one has. (This guy walked in this week is in his 20's. Time to remove it, don't ya think?) You can see it has hair growing on it. Sometimes there's bone and teeth and cartilage and other Frankensteinian components in them. Growths like this that consist of tissue not normally found at the site are called "choristomas". One choristoma made it to the big screen in the movie, "My Big Fat Greek Wedding," where the Aunt is telling the fiance of the lump that was removed from her back that had teeth and hair in it, and that it was the remnants of her twin that was never born. It was a great scene, and a proud day for choristomas everywhere.
Often, ocular dermoids go deep into the substance of the eye wall, so despite it's "stuck-on" appearance, you can't just slice it off, (or lacking instruments, pluck it off), because you could end up with a hole in the eye. So you have to have donor tissue available to patch up the hole at the time of surgery. They occur in about one per 10,000 people.
Hello, I myself have an ocular dermoid that I've had since birth. Mine is not as prominent as the one in the picture. It is also off to one side. However, I have thought about having it removed over the years. My doctors have never really noticed it growing, so they have just said to leave it be. I never realized how complicated a surgery this is, and that they tended to go into the depth of the eye. I do have hairs that grow out of it occasionally (didn't start happening until I was older), which I find very annoying. The hairs are the main reason I am exploring the topic of removal again, as they can be painful.