Q.What Is The Single Stalk Growing Up Above My One Year Old Biugainvilla Bush?
It has baby leaves coming out on it. It is 4 feet tall above the bush.

Certified GKH Gardening Expert
It looks like a wayward stalk or perhaps something that had been growing there before the bougainvillea. You can prune the stalk back by about half to keep it in line with the rest of the plant. If the leaves look different from the bougainvillea and you think it is a different plant, you can cut it off at the ground or try to dig it up.
https://www.gardeningknowhow.com/ornamental/vines/bougainvillea/pruning-bougainvilleas.htm
https://www.gardeningknowhow.com/ornamental/vines/bougainvillea/bougainvillea-garden-care.htm