Scylla

Viral video explained:
They are clumps of annelid worms, almost certainly tubificids (Naididae, probably genus Tubifex). 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.
(The link above is tame, push play at your own risk!)