It's happened twice in a row now. Can anyone recommend how to prevent it. I've had years of no fermenting problems, now twice in a row. I'm sad.
Eeeew, it looks so for me. Harmless but untasty. Scum it off carefully, and taste the brine - if it tastes OK or off.
Typically I avoid kahm by using 2% salt in the ferment, and an airtight lid. The lid doesn't need to be fancy, just to let the carbon dioxide out without the oxygen back in; for me it's typically just a plastic bag filled with water over the ferment (inside the jar), it also works wonders against mould (as the pieces of cabbage can't touch air, there's no solid surface for it to start developing).