Slow-cooker burns mash potatoes instead of keeping them warm
That just looks like mildly burned mashed potatoes to me, possibly with a little oil/margarine included.

I think the issue here is just that potatoes are thick, and there was no stirring. Slow cookers are best suited for liquids, so that convection can transfer heat throughout the contents. With potatoes, nothing can move around, so whatever's around the edges can get a lot hotter. I've definitely managed to burn things a bit in my slow cooker too, for example apple butter that was too thick for good convection.

I'm not sure there's really much you can do about this given your slow cooker. If it had an even lower "warm" setting, you could try it. If you were willing to make really wet mashed potatoes, there might be enough water to avoid burning. But otherwise, you might have to look to another solution for keeping warm, like the oven, or or maybe an insulated carrier/box if it doesn't need to hold too long.