These things recently appeared in my tap water Copenhagen Denmark, this summer 2013. They are very small, and have a feather like structure. I've called the water supply, and they thought that it was some tracer put in there by humans to trace if the warm water supply leaks into the tap water. But I could not show them the pictures.
What are they? Are they dangerous?
For size comparison: the green stuff is located on the top right of a piece of A4 paper.