Why did the grout turn white on my slate patio?
Most grouts take three days to fully cure and even then you're warned it might take longer; you only waited two.... then you power washed it.. big mistake. You removed the top coat of grout and let it set on the tile. That's where the haze came from. Get some grout remover from your home store and clean off each tile, being careful not to get in the grout lines. Where the grout lines have turned almost white, you'll probably have to remove some or all of the grout and add new grout. You might luck out and get a close match grout stain from a tile store and apply it to your grout.