Um that's perfectly normal.

I have around 10 tube anemones in my tank right now. One is over a foot long. All of them retract from time to time, especially when food hits the tentacles.

You really want to have something to worry about? Watch it crawl out of it's tube and set up shop in a new location. I've seen it several times. Takes about 2 weeks and you'll see a new skin start to grow around them.