It may be salts in your water that have accumulated in the particle matrix. It's best to either use distilled water or like I do put the stone in a mason jar filled with water and keep it that way. If it's salt residue it's because the water you put on it dries out leaving any mineral behind. I know I've read whether that can be harmful or not but I don't remember anymore. Maybe it is and maybe that's why I decided to keep mine immersed all the time. There may be some info on Lee Valley's site about it.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.