What are these white spots

[Wiqisi]

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

 

[Dave]

Can you upload a picture directly to the forum. The image on that site is poor

 

[Dan]

The grout isn't too clever in the top of that image.
What tile type are they?