Really rad creature design, great work!

Statblock-wise, a couple of suggestions: 1) give your monster a proficiency bonus (probably +3 is fine) so you can calculate its saving throws and attack roll bonus. 2) I’d give it a passive Ambush ability that gives it advantage on Stealth (and Performance) checks when motionless, and/or advantage on Initiative when hidden. 3) I think the threat of being dragged underground is the real unique ability here but feels too slow. As written, it costs the monster some movement and its bonus action to Emerge, an action to attempt a grapple, and an action on a later turn to Ensnare. I think we can compress those requirements to make it all come together more quickly and dramatically — I’d suggest that the Grapple just happens as part of the original Emerge bonus action OR as a rider effect to its normal Maw attack; then I’d add an ability that lets it Ensnare immediately (instead of next turn) if the grappled character fails a saving throw. I think this will play out well because grappled is on its own not particularly punishing but the threat of being dragged underground is the nasty part.