They're small tube worms. If you look close at them in your tank, you can sometimes see the feathery fans they use to collect food out of the water. They're related to feather duster worms, but they never get any bigger than the size you're seeing them.
They don't bother anything, other than multiplying quickly and making it necessary to scrape them off the walls of your tank. If anything, they're filter feeders, so they will help filter small bits of food out of your water.
As far as i know the only way to get rid of them is to clean your tank out and start again.