Do you use a garden hose to fill your brew kettle? If so, I would bet those things came from inside the hose. Especially if the hose is left outside with water in it.
Just a thought because I have seen similar strings from my hose when I fill the kettle. I usually let the water run for a little while now before filling the kettle.
I forgot to update this...I finally figured out what it was! I was residue form my brew kettle. I noticed that there was a whitish film of something on the bottom of my brew kettle. This is from not scrubing after brewing...only rinsing. As I cleaned the kettle it was obvious that the mystery stringy things were part of this film that broke free during boil.
No it was just accumulated film from prior brews. I would just rinse it out and call it good which left a film of residue on the bottom that was very hard to detect until you actually scrubbed on the kettle