I'm okay with dead arms, gives me an excuse not to do other things.
Google was adamant that water could soften it, and while it does soften it not enough to help the large chunks be scraped off like it says. But the "concave" bits of adhesive (might be that they used grouting only?) does soften... So I'm going to chip off the worst of it, then spray some water on the bits that remain and run over with a sander to smooth everything over including the wall underneath.
Maybe if i pressure washed it with water there would be more affect, but that would create too much mess... Rather some chips on the floor than a stream down a passage.