Mold is a negative influence that can grow on the log (except in Mushroom Garden Seasons which doesn't feature any mold) to clear away mold simply tap on it until it disappears. If the log is allowed to remain moldy for long enough it will become sick and require you to purchase a Disposable medicine in order to heal it.
In Mushroom Garden Deluxe mold grows out of a specific hollow in the log and you will often get rewards such as NP and sometimes things like Fertilizer for removing it. Deluxe also has a greater range of mold, including normal mold, big mold and red mold. NOTE: The type of mold growing does not change, if you have normal mold it will not change into a big or red mold if you leave it long enough.The Funghi (upper right corner) called Moldy is the only mold related thing in Seasons.
In NEO Mushroom Garden Mold directly attacks Funghi and puts them through various phases, usually when the mold inhibiting property of food has run out for a period of time. Phase 1: When Mold are attached, Funghi are healthy but cannot be picked until the mold is cleared. Phase 2: Funghi become diseased and turn "plain" and purple, the disease spreads to neighboring funghi. All diseased funghi cannot be picked while diseased and return to their previous form and can be picked when the mold is cleared. Phase 3: The funghi becomes a Withered, funghi has "died" and can be cleared. If there are still funghi on the board, and no mold inhibitor the mold will continue to spread. In general, the rarer the funghi, the more likely mold will attack it. Other than saving dying funghi, there are no rewards for clearing mold.