Confidence: It's likely to be this, but I can't be certain.
Notes: It looks like a scale worm with 15 pairs of overlapping scales (some missing) with a dark spot in the centre. The middle of the back is not exposed. I'm not sure if you can ID it to species level from the photos. I was thinking maybe Gattyana cirrhosa, but apparently this species lives inside the tubes of other worms and in the photo it seems to just have been under a boulder, not in a tube.