It's really hard to see it in the picture... is it the large white area or something gel like growing around the moss? It's rare for mosses to get fungus and the moss looks healthy so were it me I would remove the moss and give it a 20 minute soak in a 1 to 4-5 part hydrogen peroxide 3%. If it can't be removed you can pull up 3ml of peroxide in a syringe to every 1 gallon of tank water. Turn off filters, slowly squirt the fungus, leave filters off 20 minutes. If it's any type of fungus/mold that should kill it off.