Yes, it's those nasty fire/ bristleworms!
I quit buying live rock as bristle worms are very destructive... They will destroy corals, attack sleeping fish, and generally wreak destruction in your tank...too much food is usually blamed for them, but they will increase in size and numbers even if if you don't have excess food, and are an concientious aquariust and keep your tank emmaculate, they will just eat your corals and start hunting your fish,[ especially hippo tangs, as they like to hide under the rock!!] snails,anemones.anything is fair game for them! But your corals/anemonies are especially vulnerable..... I lost a $1200.00 12 inch "show size" red haddonna carpet anemone 5 years ago to them. They managed to get under the anemone and slowly started working on it till it was so torn up it was beyond saving as bacteria started to dissolve the damaged tissue.. Haddonas pack a lethal punch, but since the worms go underneath ,the anemone was helpless..it was gone[ 2 weeks] before i finally figured out what was going on....they retreat into the rock at dawn, and you usually don't have a clue about them as they usually travel under/through the sand/gravel...you can use many types of eradications, but that will usually cause a chemical/bacterial imbalance on the rock, which starts the next phase of problems. Geez, sorry about the long tirade, i hate them lil buggers, can you tell?lol!!!!!!!!!!
You can get the specialized crabs to work on the population of worms, but they will eat you corals too.... I'm working on an project wherein you take an appropriate sized dried krill, soak it or inject it with antipasa-x solution and put the krill in the bristleworm trap. The idea is that the worms ingest the chemical and will explode..... Anyway, get out your tweezers!!! And watch out for stings from them as some people are allergic... I have seen these worms get to the size of your small finger and as long as 8 inches or more. The live rock is porous, so they have a great "alien" travel ticket!! If you are seeing small ones out, you will find "mama" deep in the rock...it's like an iceburg....................mmm, material for a new sci-fi movie.....lol- cindy @ awesome aquariums