Yeah, and it may sound dumb, but be sure the tips of the wash arm aren't being held under the edges of the tub wall and preventing the arm from being lifted up and off.

Yes the arm should just lift off but the bronze sealing ring often wears into the Bakelite base of the arm and blocks easy removal. unfortunately sometimes when it is really stuck you will end up breaking part of the wash arm support when it finely comes loose. If it is really stuck you can pull it up a little and spray silicone spray under the edge of the hub and on the SS shaft this might help.

Important note to all KA DW owners remove the lower arm on a regular basis, you need to do this to clean the filter anyway, this will keep it from getting stuck. The filter needs to be cleaned every few loads and make sure that the DW is getting enough oil in the way of dirty dishes. The worst thing you can do to any DW is to pre rinse the dishes, DWs are not designed to wash CLEAN dishes. KA DWs had a beautifully built SS shaft and bronze bearing in the wash arm that will last the life of the machine if the machine is used as designed to wash DIRTY DISHES.

The arm was stuck on the bronze ring, it wears into the base of the bakelite wash arm hub. Its getting stuck has nothing to due with mineral build-up and once they are this stuck you need a wash-arm and new wash-arm support anyway. See my earlier post as to why this happens. If you look at the white nylon piece where your spatula is, that part is broken loose and will block most of the water that usually goes into the lower main wash arm. And unless you can think of a clever way to fix yours it will need these parts, unfortunately they are NLA although you may find some on line or Ebay. I have good used parts if you want to contact me.

Easy explanation.  It's a KitchenAid.

Seriously, though, it may be OK for now, but the issues with the wash arm could eventually cause the problems described above unless you replace the necessary components.