Is potassium acidic or basic

As the elemental metal it is neither basic nor acidic.

However, when combined with oxygen ,as potassium oxide (K2O) , then K2O is a BASE.

Since both the metal 'K' and the base 'K2O' will dissolve in water to form the ALKALI Potassium hydroxide (KOH)

NB An Alkali is a soluble base.