Holy Cats Mystic
Well forget the traps, well at least the ones I have seen, it won't fit!!!!!!!!!!
It is really two feet long?
IMHO, if it got to be two feet long, and when they are small they only feed on detritus, you maybe feeding too much even if you only feeding once a day.
If you really believe that it is in the rocks, you could do what the folks who sell live rock, when they perpare it for shipment.
They put the rock on a shelf of some kind, above a bucket, pan etc, and squirt water (salt) on it, I don't know if a hand spray bottle would work to keep the rock damp enough to keep the good critters, but anywho, when the worms figure they are in a bad place they will bail out of thier hidding places in the rock and fall into the bucket!
Then you can sort out which critters you want to keep!
Your substrate maybe hidding some also, so you might want to look there too.
When you do catch get a picture that will great to see that sea monster!