What kind of worm is this?

When I finished watering my trees a little while ago I noticed a worm crawling along the rim of one of my pots. He was long and thin and at first glance I assumed he was an earthworm.

Then I realised that what appeared to be a large piece of soil at one end was in fact his head.

I wanted a clearer shot but he started moving into a position where his head was hidden, so I moved him onto a brick where he was more visible. The lighter background showed off his stripes a lot better too.

Aside from his strange appearance  something I found odd was that his length seemed to be variable. When I first noticed him he was fairly short, then, while I fetched my camera, he stretched himself out along the pot. After I moved him onto the brick he contracted again.

I have no idea whether he could do harm to my trees so I'm hoping somebody will be able to identify him for me.

I'll update this post if and when I get more information about him.