Hey there: Most likely you are right about it being a fungus. Without a better discription I can't be certain which one. Many are harmless and all you need to do is remove them before they mature and puff spores all over the place. Some of them grow on damaged or rotting wood so it is important to check the roots of your plant to see if that is the problem. If the roots are rotting, remove all affected wood and treat with a fungicide. Correct the condition that caused the rot and replace the soil with soil that has been through the microwave treatment. Be careful in the future not to overwater or leave the plant sitting in water. Good luck-Sandy