Unfortunately, that appears to be a prolapse. The intestine has dropped through a weak point in the abdominal wall and been pinched where it hangs.
If you stop feeding the fish, the intestine may go back up again, but in all likelihood, it will come out again at a later point, probably sooner than later. I hope another eye can spot something more encouraging than I can there, but I have had this happen with fish and none survived it long.
If I am right, I'm sorry to be the bearer of bad news.