Diagnosis
That's mineral scale (likely calcium carbonate) from water condensation or occasional drips evaporating on the stainless steel exterior. It's purely cosmetic and common if you live in a hard water area.
How to Fix It
Use a mild acid to dissolve the scale. White vinegar on a cloth works perfectly. Wipe down the affected areas, let it sit for a minute, then rinse and dry. For tougher buildup, you can use a paste of baking soda and water as a gentle abrasive.
If That Didn't Work
If the spots are actually tiny pits in the stainless steel (unlikely with food-grade 304 SS), it could be chlorine or fluoride-induced corrosion, but that's extremely rare. The chalky film is almost always just surface scale.
Heads Up
Don't use harsh abrasives like steel wool or strong chemical cleaners on the stainless steel. You'll scratch the finish and make it harder to clean in the future. A regular wipe-down with a damp cloth after filling prevents this buildup.