Cleon
Legend
I don't really have a strong opinion one way or the other about the "tubular speed", but what's the logic behind it? If it's because it can propel itself from more surfaces, then a climb speed would seem a better fit. If it is because of water jets or somesuch, wouldn't any creature in the tube gain a swifter speed? If its supernatural, then it would need to become a magical beast.
Well the original Lankhmar version clearly states "Movement: 12 in coral, Sw 3" but neglected to explain the mechanism as to why it moves quickly through coral (which I presume to mean moving through the tube-complex of a giant coral reef rather than burrowing, since there's no mention of a burrow speed). Regardless, it infers that the coral worm is specially adapted for slithering quickly through its coral tubes.
If I were to hazard a guess, the "slug like" coral beast has a specialized mollusc-style foot that goes all around its body, so when it's in a tube it can simultaneously push against the walls and ceiling as well as the floor to move itself along. When in normal terrain (e.g. outside the reef) it can only push against whatever surface it's lying on and is reduced to the speed of an ordinary giant snail.