Certified GKH Gardening Expert
This could be a parasitic plant, a plant that derives its energy by feeding on other plants. Dodder is a yellow-stemmed twining parasitic plant that has white flowers. If it's dodder, you will be able to see its yellow vine-like stems.
Or, it could be an ordinary vine which has climbed up the holly. In this case you would see the stem of the vine climbing up the holly from the ground. Hopefully it's not poison ivy, which has white flowers!