Tunneler. The worm can burrow through solid rock at half its burrow speed and leaves a 3-foot-diameter tunnel in its wake.

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) piercing damage. 

Acid Spit. Ranged Weapon Attack: +8 to hit, range 20/60 ft., one creature. Hit: 16 (3d6 + 6) acid damage.

Wretch Acid Pool (recharges on a Short or Long Rest). The yellow worm spews acid in a 10-ft cube, centered on a point within 10 feet of it.  The pool is difficult terrain.  Any creature that enters the pool for the first time on a turn or starts its turn there must make a DC 16 CON saving throw or take 10 (3d6) acid damage.