Stop Arduino from powering LED strip when connected to USB
I'd try powering the led strip for the power-supply directly, and the arduino board via a (schottky) diode. 

That way the led strip will have no power when only usb is plugged in.

The Arduino won't mind that it receive a bit less that 5v because of the voltage drop of the diode.

The only thing to look out for it sending a 5v signal to a WS2812B chip that has no power. According to the datasheet this is not allowed. 

So you either need to buffer the output of the arduino going to the input of the WS2812B (e.g. using some transistors, or a simple pull-up with diode). Or, since you are not hot-plugging the usb, have the 5v from the power supply go (via a resistor) to an input on the arduino, so you can detect this in your setup routine, and have it block the rest of the code, or something.