for the tree to have a negative impact on the wall depends on a number of factors, including whether the tree is a high, low water demander, proximity to the wall, foundation depth and soil type. Trees may damage structures either directly or in the case of subsidence, indirectly. For subsidence to be an issue then this is generally on clay soils on the higher end of the plasticity scale and after an extended period of dry weather.
If the tree is a 100 year old magnolia growth is likely to be incremental year on year, if you haven't seen Ny sign of damage so far then I would not be overly concerned. This is all said without seeing the tree or wall so treat as a general rule.