So I've been breeding guppies for about a year but have never encountered this. This batch was born about 2 weeks ago. I thought maybe it was part of its egg sack and it would eventually fall off but it's still there. It's pretty thin but it's not a fin. He's acting normally. Eats good. Swims fine. Very active. But I can't figure out what that is. Should I be worried? Sorry if pictures are bad, my phone doesn't focus very well.