Sometimes potatoes will turn grey after you peel and cook them due to the process of oxidization when the peeled potato is exposed to the air.
When the potato is peeled and exposed to the air the potato gets oxidized and turns grey or other dark colors because of the ferri-chlorogenic acid that is oxidized due to the oxidization in the boiled, fried or other cooked potatoes.
Even when the potatoes turn grey or other colors after cooking the potatoes are still safe to eat and the discoloration and grey colored potatoes are harmless to eat.
They may not look as appetizing but the grey potatoes or other discolored potatoes are not harmful to your health.