Probably terrible, but it depends on your charger and the relay you're connecting it to.
Will you be connecting anything to the actual phono signal wires? Rather don't do that...
Try it out laying the equipment on top of the turntable, near the tone arm, and see if you can hear it. Especially when the relay clicks.
It might be better to use a linear power supply, not a regular usb phone charger. You can buy them new; for a one-off I would hunt in thrift stores, either for loose wall warts or old cordless telephones, bedside lights, radios etc. The one you want feels like it's full of iron, not light weight like a modern phone charger. Be aware that the voltage output will be higher than it says on the power supply.
Suppression and shielding of RFI is a whole big subject. Try to keep the interfering circuit far away from the audio signal wires. If you can do the relay switching down at the power plug, for example, not in the turntable, that would help. Avoid any loops of wire, keep the supply and return wires twisted.
Grounding and shielding are complicated - the best would be to come back and ask another question with photos etc, when you have a specific interference problem.