Since most Chinese Peonies are grafted, I would remove the sucker by inserting a sharp blade just below the sucker to sever.

then if you have removed it pot it up and see what the stock parent was like.

before cutting I would gently remove the soil and see if this is coming from above or below the graft point. if its above it leave it alone it is the same as the rest . if its below its stock.

Thank you both for your help :-)