ᐅ Sandstone slabs with efflorescence and pitting corrosion in metal flashing

About 10 years ago, our terrace was renovated with sandstone paving. There is also a small covered area. In this section, we constantly struggle with white efflorescence or residue. The landscaper advised us to treat the stones once a year with an outdoor cleaner for sandstone. The outdoor cleaner works very well to remove dirt caused by algae and similar issues, but it does not remove the efflorescence. Using a deep cleaner (also recommended by the landscaper) did not help either. Additionally, some spot corrosion has appeared on the metal flashing installed in this area. Is the corrosion caused by the outdoor cleaner or the efflorescence? And how can we remove the efflorescence or white residue? The area that is irrigated is in perfect condition.
Thank you very much for your help.
maxi.12