However, the puzzle is hopefully no more as on the Deep Sea News Website a Dr. Timothy S. Wood, an expert on freshwater bryozoas, says they are clumps of annelid worms, almost certainly tubificids (Naididae, probably genus Tubifex).
Dr Wood goes on to say, Normally these occur in soil and sediment, especially at the bottom and edges of polluted streams. In the photo they have apparently entered a pipeline somehow, and in the absence of soil they are coiling around each other. The contractions you see are the result of a single worm contracting and then stimulating all the others to do the same almost simultaneously, so it looks like a single big muscle contracting. Interesting video.