The Goblin and The Dancer by Allison Tebo

Grik the goblin spends his days as a janitor cleaning the Metropolitan Dance Hall, drawn to the Elvish world and tired of the darkness of his underground home. He secretly pines for the ballet company’s lead dancer, Rosanna, but his own ugliness and shyness stand in the way of confessing his love.
When a handsome soldier named Paul appears on the scene to make a bid for Rosanna’s affections, Grik’s jealousy bubbles over and he commits the unthinkable, plunging all of them into the river and down into the depths of the earth.
Determined to redeem himself, Grik leads Rosanna and Paul through the place he calls home: but even a goblin can’t be prepared for everything that is found underground. Trapped in the dark, nightmares both within and without rise to the surface, threatening to destroy them all.