Do you have a close up pic of it when it's on your sandbed? When you mentioned it has red hair, I'm thinking whether it's some type of tube worm or feather duster.
in your first pic, it looks like regular bristle worm..
In any case, both types of worms tend to muliply in nutrient-rich environments with lack of natural predators. Both are beneficial organisms which help to cleanse your water either by filtration or consuming detritus