Getting AC potential in PCB GND
What might be the cause of the generated AC potential?


To meet EMI limits on the output DC voltage of an unearthed power supply, usually a capacitor is placed between one of the DC output lines and the AC rectified bus voltage on the primary. It isn't perfect but it does the job. See this example here of the internal working of a wall-wart: -



Picture taken from here.

And, the impact of adding this capacitor is that it couples the input AC voltage to the output DC terminals but in a safe way so as not to risk electrocution but, if you looked at the DC output your would see a voltage waveform at the same frequency as the AC power. Of course shorting the DC output to "real" ground will remove this safely.


  What impact will it have on my PCB?


Probably no impact but, the devil is in the detail and we have no detailed knowledge of what your circuit is or how you have precisely impemented the grounding.