Do you want your fuzzy guy to look like this? I recommend to clean a tarantula's enclosure from time to time monthly. Not only different kinds of fungi but mites and other parasites would infest over the substance over time. This isn't just tarantulas, please clean and inspect monthly for infestations. To fully get rid of an infestation, you not only have to clean the substance but also doing a deep clean of the container as well. If it is a fungi problem, stop spraying water into the exhibit so often and clean the tank more. There are signs you should spot like mites crawling everywhere in the enclosure, how the tank is foggy with too much humidity or how the tarantula isn't moving much.