What caused stains on treads on outdoor steps?

Just noticed today that we have many oval white stains (similar to ceiling leak stains) on our sandstone treads on our front door steps. The house is in New England and is 6 years old. Didn't happen when it snowed last week or in the past years. It is not salt ( haven't used any) and not the red iron stain typical on the concrete risers. It also does not show on our top brick tread or on brick walk. Does anyone know what this could be from? Is this normal for treads to get stains? How do we get rid of it? Any advice would be appreciated. Thanks, Lisa
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