In Dragonflight, Fluorescent Fluid is an item that you need to get Lizi's Reins mount and do a particular quest sequence.
Fluorescent Fluid is required for To Tame A Thunderspine Quest. You can get Fluorescent Fluid from any insect in the Dragon Isles. It is also quite possible that you already have some in your bags.
If you have to collect it, then you can find some caves full of spiders in the Azure Span, for example just east of the Azure Archives. There you’ll find many spiders,
Once you have collected all your fluids, you can continue the questline.
How to unlock and advance the Lizi Reins quest?
This questline requires you to have Renown nine with the Maruukai Centaur to unlock Replacement Hooves and to have completed the quest Initiate;s Day Out. Then you can find the Initiate Radiya to start the questline.
Go to her every day for a new quest until you get your mount. All quests have the same name, To Tame A Thunderspine, but ask for different items each day.
First day: 20x Fluorescent Fluids
Second day: 20x High Fiber leaves
Third day: 10x Thousand-bite Piranha
Fourth day: 20x Woolly Mountain Pelt
Fifth and last day: Meluun’s Green curry
Once that's done, you can get Lizi Reins mount.