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I'm sorry, but I won't be able to offer too much information without knowing more about the type of fruit tree, and having some photos of your issue. All that I can do, for now, is offer articles on fruit trees and their care:

https://www.gardeningknowhow.com/edible/fruits/fegen

To safeguard fruit tree blossoms from unexpected frost, cover them with frost cloth or blankets secured to the ground before the frost event. Alternatively, use heat sources like incandescent bulbs or Christmas lights under the coverings for added warmth. Watering the soil beforehand helps retain heat, while windbreaks and anti-transpirant sprays offer additional protection. Pruning dead branches minimizes vulnerability. These measures help mitigate frost damage, ensuring a healthy fruit harvest.

It will, likely, kill or reduce the growth of the intended tree. Below the graft scion is a rootstock that is fast and hardy, but will not bear fruit of any quality. It is, simply, used as a hardy rootstock for the top grafted portion of the tree.

In short, leaving that sucker will likely destroy your tree over time. You could separate it and root it, but you may find that it is an invasive, and annoying version of the one you purchased.

Also worth noting- Seeing these often is a sign that your tree is stressed. Unfortunately, your photos did not come through, so I cannot see if there is anything readily visible.

https://www.gardeningknowhow.com/ornamental/trees/tgen/tree-sucker-removal-and-tree-sucker-control.htm