Impregnaters only , no topical sealer in wet areas, they can go white as give issue with slip ( as chalker). Try a colour enhancing impreg to take the dusty effect away, if you have it
As chalker, on the white marks, best way to stop these is to rinse after use, towel dry after also if you want to really keep them looking nice. Water and diluted shower products will sit on any impregnator and evaporate causing these marks/ mineral salts. Most important thing , use a stone specific cleaner for maintaining