Rhizoctonia on staghorn fern
Without getting a sample of the infestation under a microscope, I can only limit my recommendation for treating fungal pathogens to a general strategy. Normally I would suggest removing the infected portion of the plant, but since it is located in an area that is less conducive to do so that may not be an option. Using a dilute solution of fungicide (take a look at your local garden center's offerings), dab at the infected area with a paper towel and try to remove some of the fruiting bodies. Follow up this process a few more times, allowing at least 24 hours between treatment. Be sure to thoroughly inspect the plant for any further spread.