Quite possibly could be a fungus. I have heard rubber trees are pretty suseptible to fungus. You can try gently brushing the spots off and trying to keep the stock of the plant more dry
White spots on a rubber tree usually mean fungus and sometimes mold. The best thing to do to treat the tree is go to a local nursery and they will sell you anti-fungal spray to put on the tree. Also, I would suggest cutting off as many leaves as possible so the tree grows stronger.