I think you have a simple problem! Basically dust or dirt has collected on the backside of the cover glass and/or the display its self.
Follow this guide to pull off the cover glass, use a dry micro cloth dust off the inside surface, then using some window cleaner and a fresh micro cloth spray the cloth and wipe down the cover glass surfaces. The LCD display should just be dusted as it has coatings on it the cleaner will either damage or just make a mess and/or streaks!
Here’s the guide you’ll need to follow Apple Thunderbolt Display LCD Replacement following Step 1
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