Bristle worms and new little creature

[BPUMFREY]

Ok so I know some bristle worms in your aquarium is well a good thing... But we have an abundance of them and I was wondering how we might lower said population any suggestions would be greatly appreciated... Also my wife is currently trying to get a picture of it but we have something that has attached itself to a small piece of live rock that has camouflaged itself to match said rock its about the size of a shell and looks like it has a mouth (opens and closes it) its not a bi-valve or a muscle the creature appears to be soft but does not move and we would have not noticed it if are hermit crabs weren't trying to attack it every time we turned around i will try and get a picture of it and post it but does anyone have any suggestions on what this thing could be??

 

[303travism]

I've had great luck with an arrow crab it eats them but there are still some in the system

 

[BPUMFREY]

Thanks but arrow crabs are not reef friendly?

 

[303travism]

Thanks but arrow crabs are not reef friendly?

I've had one in my reef for over 6 months now with no I'll effect I had a bigger one before but sold it when I thought the worms were gone, I'll just hang on to this one it just hide in the rocks only see it about once a week which is fine with me kinda look like a creepy spider in my opinion.