Movement is normal under expansion and pressure.
You might want to top up, engine cold, to the top mark. Cap off, heater on full, run the engine hot enough for the thermostat to open.
You may have a small airlock or a leak. Observe safety precautions. (Also, when done and have secured the cap, keep an eye on the level during your motoring). Engine off: take top cover off and with a torch have a good look for damp patches or crystallised debris of coolant. Check hoses and clips.
Bit belt and braces, but this is worth a quick check and slowest part is awaiting the engine to warm!