Liverwort. Nothing kills it. Not flame gun, not weed killer, not moss killer. Apple cider vinegar was recommended, but that did not work either.
Scrape it off.

Thank you OWD for the name and your experience. We also tried weed killer and torch without any success. Now that I know the name I will try to get some more info on it.
Thanks again .

Don't put it on the compost heap either.

This is a nonvascular plant so it is difficult to control. On the brighter side it is also one of the first colonizers on hard rock substrates helping to form the most precious of all substances....soil.

Thanks Loosestrife for that info but it does not make me to dislike it any less. Any idea how to get rid of it? It is very resilient.

Physical removal ...repeatedly so....as Owdboggy has recommended.

I have it in my garden. I remove it by hand sometimes but covering it with a good layer of soil helps, as like all plants it needs light to grow. I don’t know where mine came from, whether from a pot plant bought from a GC or brought in by the birds. I suspect the former.