What happens when sodium or potassium are left outside in the open?
The surface slowly oxidizes and degrades.
The alkali metal will react with (i) air-born moisture, and (ii) atmospheric oxygen.
Because this is a heterogeneous reaction, this process is slow. Often a lump of waste metal can be discarded in the trash, and when it hits water, potassium metal will cause ignition of whatever is combustible.
Certainly, potassium residues must be dealt with properly. If you need advice, ask!