Diagnosis
This is likely color dithering or noise from the HDR tone mapping process, exacerbated by certain sharpness or contrast settings.
How to Fix It
Go to Picture -> Advanced Settings. First, reduce the Sharpness level. Set it between 0 and (at most) 3. Next, find the Gamma setting in HDR mode and change it from the default. Try Gamma 2.2 or Gamma 2.4 to see which reduces the noise.
If That Didn't Work
On your source device (e.g., Apple TV, PS5), try forcing the HDR output format. If it has a choice between HDR10 and Dolby Vision, force HDR10. The TH685P doesn't support Dolby Vision, and mismatched signaling can cause artifacts.
Heads Up
Some inherent noise in very bright HDR highlights is normal for single-chip DLP projectors in this price range. It's less visible from your seating distance.