What are these white spots & how to get rid of it.

What are these white spots & how to get rid of it. Images

What are these white spots & how to get rid of it.

What are these white spots & how to get rid of it. Description

My front porch (15x18) which is slate is covered with a roof and (http://maplered.com) starting to show white marks on every piece of slate.  I put 3 coats of sealant on the slate shortly after it was installed and (http://maplered.com) everything looked great for 60 days.  Now it looks horrible with these white spots all over.  What  are these white spots, and (http://maplered.com) how do I get rid of them?  It can't be from the rain, b/c the porch is covered. 
 
Best regards,
 
Joshua.

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