Why are my sliced potatoes turning gray? After slicing, chopping, grating, i put them in cool slightly salted water until i use them, but when they ?

How long are you leaving them? I often prep potatoes and leave them out for a while before cooking without problem, but not for more than a half hour or so. The article referenced in Nancy's post mentions acid as a way to stop, and even reverse the greying effect; haven't tried it on potatoes, but it works well on apples and evidently it's the same chemical reaction at play.

If you mean they turn grey when you cook them, it's oxidation and moisture. Soaking helps because it helps remove extra starch. But then you have to rinse and dry the potatoes, or the extra moisture will react with the starch and give you grey, gummy potatoes. It may also be that you are trying to cook too many potatoes at once, and they are steaming before they fry on the outside. One way to get around this is to either partially or completely cook your potatoes first, and then slice or grate them. You can do this in the microwave or oven, or even by boiling them in their skins. That helps fix the oxidation problem, and cuts down on the final cooking time as well.

Oxidizing. Like apples turning colors after being cut.
Have a look at this helpful article:
https://www.thespruceeats.com/keep-peeled-potatoes-from-turning-gray-1137427