Update: Thanks to my awesome commenters, I learned these are called "Milk Spots".
"Milk spots are an inherent part of the manufacturing process. Before being stamped with the design, the blank or planchet is cleaned using a variety of chemicals to help ensure as high quality a finish as possible. The coin is then heated and cooled – annealed – to help ensure it does not become brittle. If the chemicals have not been fully removed prior to annealing, it can leave a residue on the coin. This leaves the coin looking perfect upon minting, but over time the residue can slowly react, causing milk spots to appear on the coin." source