Hi Ski,
Ah thanks! Yeah its been moving around a fair bit. The funny thing was, I saw its tenticles when I was looking at the rocks when I first got the tank. I thought they looked "odd". I said to my brother "I swear there's something in that rock". I then rearranged the rocks and even had my fingers inside that one as I was holding it. Good job it didnt move and touch me, or I'd have jumped, dropped the rock and smashed the tank lol. I thought it was a crab at first, because it was so tucked away I couldn't see anything more than the tentacles. It wasn't till a few days ago that it moved out more so I could take the photo.
Thanks again.