there perfectly edible , but if there green its said to cancer causing. though i,m no expert

As said, they are perfectly edible. The reason for not wanting to let them grow as opposed to eating them is that they can harbour the spores of Blight so you get the infection from them.
The green part of potato tubers which have been on the surface is poisonous, but the rest of that tuber is edible.

This happens in my garden every year and we always eat all of them.(Except the ones we inevitably miss again...) Some old gardeners call them volunteers.

We always end up with a few plants on, or beside the compost heap where dried up or halved potatoes have been discarded. If there are potatoes, it's a free meal - enjoy them. Potatoes turn green if in the light and the peelings are poisonous, though you would probably have to eat a lot. We are (hopefully) not in the position of the Irish during the potato famine when some died from being forced through starvation to eat green peelings from their betters - or is that an urban myth?

Urban myth, they died because the potato crop failed utterly as it was infected with Blight and so the tubers rotted in store. It was made worse by Parliaments refusal to repeal the Corn Laws which meant that the price of bread was way beyond the income of the lower classes.

However I did once read a theory that eating green potatoes contributed to the incidence of spina bifida.