I'm messing with you because it's an English yeast, suggesting you use what is effectively the British English equivalent rather than the German word that has been borrowed by American English.
But "barm" *is" the yeast in the Anglo-Norse view of the world, rather than the action of the yeast. It's a bit old-fashioned, you only see it commonly used outside the traditional breweries in the phrase "barm cake", which is the Lancashire name for a bread roll/cob/bap.* But it clearly has the same origin as "berm" which is what people around the Skagerrak (Denmark/Southern Sweden/Telemark) call their yeast instead of kveik.
*the ethnography of naming small lumps of bread in the UK, particularly northern England, is fascinating. It's perhaps the most sensitive way other than accent to work out where someone was brought up.
View attachment 816455