Hi @Shalarbut
This is actually the PWA's simpler version of the virtual background feature, called the virtual mask. Since this is simpler than the full virtual background feature, it requires less processing power and allows for something similar to virtual background on the PWA.
You can disable this by selecting the Settings option, switching to the Background tab, and selecting None for your background.
We did recently release the ability for the PWA to also use virtual background without a greenscreen, as seen in the PWA release notes, but this will only work smoothly for devices with sufficient hardware. I'm not sure what type of Chromebook you have, but you may be able to use this instead.
Hope that helps and please make sure to mark the solution as accepted if this information is what you needed.