Almost everyone's first ferment produces a moment of doubt. Something looks different from yesterday and there is nobody to ask.
Here is the short version: fermentation is one of the safest forms of food preservation there is. The acid environment that develops in the first few days is actively hostile to the organisms that make people ill. The things that go wrong are mostly cosmetic, and the one that is not is unmistakable.
Normal, do nothing
Cloudy brine. Expected from about day three. It is lactic acid bacteria doing exactly what you wanted.
Bubbling, hissing, the lid burping. Carbon dioxide. A water-lock crock is designed to vent it. If it stops after two weeks, that is the ferment slowing down, not dying.
A sour, sharp, slightly cabbagey smell. Correct. It should smell like a pickle, or like a very tangy coleslaw.
Colour dulling. Bright green goes olive; red cabbage goes magenta then pink. Normal.
Brine rising and occasionally overflowing. Very active ferment. Put a tray under the crock.
Cosmetic, deal with it
A flat white film on the surface. This is kahm yeast. It is not mould. It is wrinkly or smooth, always flat, always white or cream, and it smells musty rather than foul.
Skim it off with a clean spoon, check that everything solid is under the brine, and carry on. It usually appears when the temperature is too warm or the salt is a little low. The ferment underneath is fine, though a heavy kahm bloom can leave a yeasty note in the flavour.
Soft or slimy texture. Often too warm, or too little salt. It usually firms up as acidity develops. If it stays slimy and smells fine, it is safe but unpleasant — use it cooked.
Stop, discard
Fuzzy mould. Anything raised and furry — blue, green, black, pink. Mould has texture and structure; kahm yeast does not.
The standard advice for large batches is to remove the affected portion. The honest advice for a home crock is to throw it out. Mould sends threads well below the visible surface, and the taste will have travelled even where the colour has not.
It smells genuinely rotten. Not sour — putrid. You will know. Trust it.
Pink slime through the whole batch. Usually too much salt, which suppresses the good bacteria and lets the wrong yeasts take over. Discard.
The single rule that prevents most of it
Everything solid stays under the brine.
Mould needs oxygen. Below the brine line there is none. The weights are not an accessory; they are the mechanism. A shred of cabbage floating at the surface is where the problem starts, every time.
And keep the water channel topped up. An empty channel lets air back in, and that is the same failure by another route.
Salt and temperature
Two percent salt by weight of vegetable. Weigh it; do not estimate.
Cool room temperature, 18–20°C. Warmer gives a fast, soft, more yeast-prone ferment. Cooler is slower and cleaner. A cupboard away from the oven is usually right.
When it is done
When you like it. There is no moment of completion — only a curve from fresh and salty to deeply sour.
Taste weekly with a clean fork. When it tastes the way you want, jar it and refrigerate. Cold slows fermentation to almost nothing, and it will hold there for months.