Spirorbid worms.
Quote:
Name: Spirorbis sp. (Spirorbid Worm)
Source: Hitchhiker on live rock
Number: Too many to count
Notes: Spirorbid worms are yet another critter that seem to make it into nearly every tank. They quickly cover the rocks, glass, powerheads, and any other hard surfaces in the tank, including snail shells. Despite this pest-like quality, they are harmless filter feeders and are beneficial, if anything.