Do you have cats? I've had that happen and it ended up being a cat hair. Another time it was a very fine strand of some sort of what I assume to be a synthetic fibre that ended up being a convenient anchor for the growing crystals. It wasn't a problem. I just knocked the crystals off of the fibre/hair after drying.
There are all kinds of things floating around in the air in houses. It's easy to get things in your solvent.
Welcome Home Mister_Niles. We've Been Waiting For You.
"Don't worry. When it happens, you won't be able to not let it do its thing. You won't have the ability to distinguish a pen from a hippopotamus"
- Art Van D'lay