What are these white spots

[Wiqisi]

What are these white spots

Hello I have porcelain tiles outside. They are not sealed.

I noticed these white spots on one tile. What could they be? It’s not bird poop.

I’ve tried pressure blasting it, scrubbing it with soapy water. Doesn’t work. As soon as I touch it with water it changes colour and you think it’s gone. Then the tile dries out and it’s still there!

Here is a photo

https://imgur.com/a/hwagGuk

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