If the white spots are random and not much, it maybe due to the quality of thermal label.

If you found spots at the same position of each label, it’s likely that the Printer head dirty. Please turn off the printer, wait for it to cool down, you can use alcohol cotton wipes to wipe the printer head clean (the print head is marked in red frame in below picture). You can buy 70% Isopropyl Alcohol wipes from supermarket, convenience store, drugstores, grocery stores and on Amazon. After the parts have dried for 2-3 minutes, you can turn on the printer again.