Probably a symbiotic micorrhizal fungus. These tend to protect your plant from pathogenic fungus and chemical exposure. I wouldn’t use fungicide on it as it is highly beneficial to the plant. I know certain plants like the japanese black pine will not grow without a certain micorrhizal fungus. Pull your plant out of the pot and look at the root ball. You will probably see what looks like a bunch of spider webs, this is the micorrhizal fungus. If you see this congratualtions, this will highly benefit your bonsai. If not it is most likely not harmful if it is not growing on the actual tree.