Thank you for your question. It looks like it may be a hydrangea. I see in your picture you have a hydrangea growing next to this. Do the leaves look similar? It could be a sucker or off-shoot of your original plants since they are known to do this type of growth. Do you see any blooms forming on this? If you do, I would not cut it back yet and see how it grows. If you don't see blooms, then you can cut it back if it's too large and you'd prefer it smaller. Here is an article from Extension for your information:

Pruning hydrangeas for best bloom | UMN Extension