How does the plastic nut come off the washer arm?
I see no screw or anything. THANKS

I did not have an adequate spanner wrench to unscrew the washer arm nut. As you can see the edge has a knurled knob leading one to assume it unscrews. It does they are right-hand normal threads and very coarse threads too like a water hose thread-count or coarser.

The plastic nut is kind of soft and not that fragile but be careful. Pre-treat the threads they are most likely gummed up with calcium or mineral build-up. Do this by spraying the nut inside and out with Lime Away and work a slender knife blade under the knurled knob.

Wipe off with a rag then try again with your hand, spanner wrench, or channel lock pliars and try again rocking the nut back and forth. If you are still having trouble as I did with my 10 year old dishwasher then spray the nut down inside and out with PAM vegetable oil spray.

While you are in there it is always very wise to unscrew the 6 screws under the washer arm, remove the filter screens and give them a good gentle scrubbing with Lime Away and warm soapy water.