LOL'd @ bobbit worm.
those are very nice, rounded bite marks... i would imagine a crab would leave it a bit more... shredded. Fireworms are possible but for one-they're super rare and I feel like the owner of the tank would have noticed a bright red worm chilling on the rocks eating coral in the middle of the day, since that's what they're supposedly known to do.
I see too much coraline on the rocks to imagine there would be an urchin in there. Starfish would still be a possibility though.
Alright alright, I'll have my go at it.
If it's ocean harvest live rock I'm placing my bet on;
1) Rapa Rapa Snail
2) Nudibranch
3) Spider/Decorator crab
4) Fireworm as unlikely as it may be
If it's man-made rock seeded with a little live rock my guesses are bacterial or fungal patch, but it could still be something above if it was seeded.