Certified GKH Gardening Expert
Your picture did not come through, but normally this is an infection from overwatering.
If you can catch it before you see damage on most of the leaves, then it can be cured. If it has progressed to almost all leaves, then you will likely lose the tree attempting to correct it.
You can try a fungicide. This article will help: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
The only good way to deal with this is prevention. If you can get it back to health, then you will want to make sure that the tree receives proper care. This is especially true in container, or indoors.
Here is an article that will help you to give this tree the care that it needs: https://www.gardeningknowhow.com/houseplants/ficus/growing-fiddle-leaf-fig-trees.htm