Grandma’s Stash is a brand new Merge Mansion feature available for some players. Every single time you complete a task, Grandma's Stash accumulates a certain amount of rewards.
After reaching the initial threshold, you can acquire Grandma's Stash and receive the rewards accumulated until that point. Afterwards, the bank will disappear. This is why we recommend that you fill up the Currency Bank with as many rewards as possible before purchasing it!