How to remove this?

A dilute mix of water and muriatic acid could likely help.

 
If it’s extensive then you’ll need to hire a professional pool tile cleaner. They use glass bead blasting to physically remove the calcium. Some use MaxxStrip (kierserite) media but that is magnesium sulfate and you really don’t want to add that to your pool water if you can avoid it.

That efflorescence suggests that water is permeating from behind the tile. That’s not a great sign as it means it will keep coming back. Sealing the tile and grout might help a little bit it’s not a bullet proof fix.
 
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