MOSFET switch not turning off load
Neopixels are not simple LEDs. They have a driver IC inside it. And in addition to supply and ground, there is data pins that are referenced to ground.
It is not correct to turn off the ground of an IC, because it still can leak supply current out of the data pins via IO pin protection circuitry when data pin is pulled low.
Don't disconnect the ground if you need to cut power to the RGB LED.