Look like some kind of Feather duster or worm tubes (vacated).
coralline algae - algae that secrete calcium carbonate in their tissues. Hard, encrusting, red coralline algae are significant reef builders in some areas. In wild reefs they tend to cement old corals together that form the reef itself.
In the aquarium it is a sign of a healthy aquarium, a visual pleasing sight as well as some filtration I believe.