Is potassium a macronutrient or a micronutrient?

As well as being an element, yes. It combines with Oxygen to form 'phosphate' which is used to build the phosphate-sugar backbone of DNA and Rna, and is used to form phospholipids, and which acts as the On/Off control factor that binds to, for one example, the Kinase family of [very large] enzymes.