Underlight Emerald is the newly added alternate color for the fishing artifact Underlight Angler in Patch 7.3. It is a reward from fishing achievement Fisherfriend of the Isles, which requires you to become best friends with the fishing masters of the Broken Isles.

On each map of the Broken Isles, an active fishing NPC is rotated every day, corresponding to each map of the broken islands. Each NPC has an item of the corresponding reputation, and each item can upgrade 75 points of reputation.

Impus on Broken Shore -  awarded Demon Noggin, need Fragmented Enchantment.

Sha'leth in Suramar -  awarded Replica Gondola, need Corrupted Globule.
Corbyn in Stormheim -  awarded Carved Wooden Helm, need Ancient Totem Fragment.
Ilyssia of the Waters in Azsuna -  awarded Enchanted Bobber, need Shiny Bauble.
Keeper Raynae in Val'sharah -  awarded Face of the Forest, need Golden Minnow.
Akule Riverhorn in Highmountain -  awarded Floating Totem, need Desecrated Seaweed.

When you increase these NPC reputations to Best Friend, you can purchase a special toy which transforms the appearance of your bobber.

PS: Only 3 requirements to get the fishing artifact Underlight Angler:
1, Your Fishing Skill Should be 800.
2, You should get the achievement: Bigger Fish to Fry.
3, You character's Level should be 110.
Pretty simple, right?