One of my tilapia fillets has tiny little black threads in it- not sure what this is? Is it still safe to eat? It otherwise looks fine, doesn't smell, etc.
Contact the source where you bought it -- right away, if you haven't already. It's not getting better by itself, and probably it's nothing and you should just cook it up. If it's a problem, they should replace it.
I second the motion. I use a lot of tilapia, and I've never seen what you describe.