Worms? growing on back wall

[Pennywise the Clown]

Over the last couple of weeks I have had a growth of some form of what I think are worms growing on the back wall of my 4 month old display. When I clean them off they seem to have a hard shell.
I have no issues with water chemistry and no algae problems at all other than the usual build up on the front glass.

What are they and is it an issue?

 

[Crabs McJones]

Spirorbid worms can become a problem if they reproduce rapidly. Their numbers tend to fluctuate with how much excess food is in the tank. Cut back on feeding if they get out of hand