To my knowledge, Mycus and Marloss is a matter of how much you embrace the Fruit.
Marloss is the precursor to Mycus, so in of itself it isn’t that much of a change. Oddly enough, some of the best ways to get Marlos is to destroy Fungal Towers/Spires, since there is a high drop chance of something. Then its largely a gamble what will happen upon consumption, but it seems like the more categories you fall into, the less likely you are to get a Marloss mutation. Be careful, since you can get a rejection which can kill you outright. Though upon joining the Mycus, you (we?) lose all non-Mycus mutations.
Potentially, if it removes those mutations that are non-purifiable as well, it might be a good last ditch effort to save a character from mutation-related death.
The wiki is inconsistent, though. One article says Marloss traits can be purified, while another says they are non-purifiable.