that is what you get when you burn an egg, in my experience. I’m not sure why it’s happening no matter how low you have the heat, though.
Is it a super cheap, all aluminum (or aluminium) pan? If it’s very thin and extremely heat conductive, you might not be able to get your heat low enough to not burn the egg.
First thing I’d try is adding some fat to the pan before putting the egg in. Even with supposed non-stick, throw some butter in there.