When should i use a mask behind a FAB button?

Right now at work we are making this amazing feature (for a webApp) but i don't know WHEN its correct to use a "mask" Behind the FAB after it gets TAP/CLICKED because in the guide they say nothing about it, all i know is:

  1. It would be better to improve readability
  2. I have seen it (The mask) in other google apps
  3. Pinterest use this behind-mask every time you click on a "PIN"

Below ill link you the Material Design Guide:

https://material.google.com/components/buttons-floating-action-button.html#buttons-floating-action-button-behavior

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