Update: on Wednesday, I couldn't see the worm at all and we proceeded with our regular water change. That evening, my other half saw it under the blue lights - it stretched across from one side of the tank to the other so I would say it was minimum a foot long!!! Thursday morning, I found a snail hanging of the rock encased in slime - I grabbed the snail and put him down the front (I'll let the hermits finish him off...) and proceeded in taking out the rock in which the worm resided. Tried a few things. There was no sign of the devil worm himself, just several bristle worms. Rung him to tell him what I did, he told me to get rid of the rock and go buy some more (yay, permission to shop!) so I went out and bought 7 pound of new live rock. Happy that the worm had been evicted I noticed this morning that the snail I rescued was happily grazing on some rock. The worm had paralysed him, but I actually rescued him in time! It took him about 24 hours to recover from the slime.
If anyone else comes across this worm a few tips from my experience:
Don't touch the mucus/slime with your bare hands - I did the first time round and my hand tingled for hours after. I was constantly opening and clenching my fist as it felt like prolonged pins and needles.
Trying to catch him with tweezers is futile. He is too fast, there is a thread on reef Central where someone made a trap and manage to catch it. I never contemplated in making one, but it doesn't look particularly taxing if you have the equipment. (I don't even own a drill)
Easiest way to eradicate is simply removing his home. I tried the break up the rock, but all I got was rubble instead of nice big pieces, so I stuck the rubble in my filtration system and chucked out the rest. I couldn't spend forever searching for him.
Hope this helps anyone who encounters the same beastie, I hope you never do...