I'm no expert, but I THINK those are strands of 'silver skin' (the tough membrane that surrounds some muscle tissue) that have been slivered by a knife while cutting the meat.
That's my thought but I didn't want to go first. I've seen worms but they looked nothing like that. Not saying those aren't but they don't look like the ones I saw. looks more like the silver skin like the piece below the one cut open. I usually remove what I can prior to grinding.