What has formed there on the surface is a yeast-like microbial fungus called Geotrichum candidum, also known as milk mould in Germany. Fortunately, it is completely harmless and also often a normal component of the kefir microflora. It can actually only spread on the surface if it is undisturbed for a longer period of time, e.g. if the kefir has spent a longer holiday in a kefir hotel or has fermented for a longer period of time without being stirred. It also has an important function, namely it protects the small kefir ecosystem against foreign germs from the air. It usually smells a bit cheesy, similar to goat cheese, and in fact the milk mould is also used in cheese production.
You can simply remove this surface layer from your kefir and compost it. If you don't mind the taste, you can safely eat it too.
If you don't want your kefir to form a layer like this again, just remember to stir it regularly, whether during fermentation or in the kefir hotel in the fridge.