if it's feathers - esp if you touch it, it retracts back into a hole - then it's actually feather duster worms.
Its a filter feeder. opinion on the forum is divided on if it's beneficial vs a pest.
my personal take is that it is a beneficial filter feeder that contributes to biodiversity and stability. In a healthy and stable environment, their population is self-regulating.
the people on this forum that's of the opinion that they are pests generally point to the fact that they spread very fast and can take over rocks. My take on that is that they spread b/c the tank's conditions ALLOW it spread - usually as a result of inadequate filtration and too much dissolved food particles. In these situations, my opinion is that it's actually good that the population explodes - b/c if it doesn't, all that excess food particles/excess nutrient would have caused more serious issues in the tank stability - the feather is just doing its job to trying to maintain stability.