my mandarin orange tree has a graft growing from it that is taller than the tree
Mandarin Orange Tree
Certified GKH Gardening Expert
The grafted part is the part that you want, as the original rootstock will not fruit like the part that has been grafted on. You will want to cut back anything that grows from below the graft line, as this will be part of the original rootstock, and will take away nutrients to the portion that was grafted on.
Here is an article that is for a lemon, but will apply to all citrus and will help: https://www.gardeningknowhow.com/edible/fruits/lemons/suckers-on-lemon-trees.htm