The effect is only activated when the drops appear, but because the timer continues, it is recommended to eat it as the mutated jadinko dies in order to maximise its usage. The doubled excrescence drops seem to have a cost of not receiving any other random drops, so for players who fight Mutated jadinkos for profit, it may be counter-productive to eat this fruit.
The fruits have no effect outside of the lair. It cannot be banked. If a player attempts to do this they will receive the message: "A magical force prevents you from banking this item." They can be changed to Common fruits by speaking to Papa Mambo.