Diagnosis
That's burnt-on product residue. The nano titanium coating is porous. Hairspray or serum carbonizes at high heat.
How to Fix It
Let iron cool completely. Use 91% isopropyl alcohol on a microfiber cloth. Scrub plates in one direction, not circles. Do not use abrasives. Wipe clean with a damp cloth after.
If That Didn't Work
For heavy buildup, use a paste of baking soda and water. Apply, let sit 10 minutes, scrub gently with a soft cloth. Rinse thoroughly.
Heads Up
Never use nail polish remover or oven cleaner. They degrade the coating and void the warranty.