What are Milk Spots on Silver Coins?

By GovMint :

Milk spots on silver coins are white, cloudy marks that resemble dried milk, hence their name. While such spots do not affect the precious metal content of a coin, they can be a concern for collectors due to their impact on the coin's aesthetic appeal. The exact cause of milk spots is unknown as there is not one single predictable cause, but they are believed to result from the minting process.

Sources Used:

https://www.coinworld.com/news/precious-metals/royal-canadian-mint-develops-milk-spot-fix.html 

https://www.mint.ca/en/bullion/mintshield