Live rock

Hi sorry I didn't know where to post about this, I hope I got the right place, im just curious about what's living on my live rock, I seem to have a very good population of amphipods but when I first got my rock it had a shell fish on it that looked like a typical cockle but very small, it's disappeared now, I do t know if it died or just moved to a better hiding spot, I also have shellfish embedded into the rock!? There's one that's empty but others are embedded but I can only see a fine bit of them?
Is this normal?