If you have your rubber tree plant in the direct sun, those brown marks are sunburn. Move your plant away from the window and into bright, indirect light. If the plant is not in direct sun the marks could be u003ca href=u0022http://houseplant411.com/glossary/leaf-spot-plant-disease-identify-and-treatu0022u003eLeaf Spot Diseaseu003c/au003e. Keep water off the leaves and don’t mist the plant.