Mossworm

A mossworm mass is a colony of Mossy Flatworms — hand-sized flatworm slime which thrives in the tidal line of the Undersea. They secrete a toxic substance through their long, hairlike tendrils which makes them dangerous to be stung by.

If a stinging flatworm is stung by another flatworm, their bodies gradually merge. As this happens enough they create a Mossworm Mass—a blob up to meters across which snags prey and shares the nutrients.

They grow along tidelines. Finding the distinct hatchmarking of their rootmarks on a surface is a near-certain marker that water once flowed there.

Unfortunately, they also thrive in pipes.