Thanks for reaching out.
Could you please check if the Ambient Mode on your Google Nest display is turned on? From there, you can adjust its brightness of the screen of your Google Nest display.
Ambient EQ is automatically enabled when you set up your Nest display. You can choose to have Ambient EQ adjust the screen color to match your environment Always, during Ambient only, or Never as shown below. If you’d like to offset the brightness, you can do so in the Display settings on your Home app. To disable Ambient EQ, utilize the Quick settings tray on the device when you swipe your finger up from the bottom of the display, and tap the sun icon.
Hope this helps.