are they pooping on the grass? because if that's the case, that's very unusual. Cats like to poop and bury their treasures, so they most often target soil. If they are pooping on the grass, then I would have to wonder if it's really cats and not dogs pooping there?

And while I haven't stepped on any cat poop, I doubt you'll get them all over your slippers as they're often quite dry, and if they've been lying around for a while, they most definitely will not be mushed unless you have very heavy tread soles. To me it just doesn't sound like cat poop (at least not the ones I've seen, and I grew up with cats).

If you know 100% without a shadow of a doubt that there are cats pooping on your grass, there really isn't anything you can do other than to remove what they like to use as their toilet.