In an episode of Family Guy Stewie is talking to Brian about this thing in his eye that floats and moves when he what it?

You're likely thinking of the episode "Road to Europe" where Stewie, while looking in the mirror, notices a "little thing" in his eye that moves around when he blinks. He's convinced it's a tiny person living in his eye, and gets very concerned about its well-being. He even tries to feed it, using Brian as a makeshift catapult to launch tiny bits of food into his eye.

This episode is known for its hilarious absurdity and Stewie's over-the-top paranoia.