Blue-Legged Horseshoe Worm

I noticed today that my horseshoe / Christmas tree worm living on a shell was on the opposite side of the tank where I last saw it. I has been moving around a bit. Since it was near the front glass I assumed that I was hitting it with my magnet cleaner. However, today when I saw it it was far away from the glass. It turns out that a hermit crab has moved into the shell. So now this feathery worm is mobile.