Scrub lightly with a dry magic eraser. You want to avoid water like the plague when it comes to leather.
I use a product called Dr. Martens Leather Balm(from Denmark) on all my leather shoes and boots. It protects from water and salt stains and got me through quite a few Chicago winters. After seeing how well it did protecting my boots and shoes I figured it was worth trying on my iPhone and iPad cases. I don't go out of my way to get my phone wet but you never know when it might get rained on or splashed.