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.

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.

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.