A thin, dusty film sitting flat across the surface of a jar is almost always kahm yeast, and it's harmless — scrape it off and the vegetables underneath are still good. Anything fuzzy, or any color that isn't white, is mold. Mold means the whole jar goes in the trash, not just the top layer you can see.
A ferment protects itself by winning a race, not by luck
Vegetable fermentation runs on a race, and the outcome decides whether a jar turns out safe. Salt pulls water out of the vegetables and builds a brine low enough in oxygen that most surface bacteria can't compete in it. What survives that cut is a smaller population dominated by lactic acid bacteria, which feed on the vegetables' natural sugars and excrete lactic acid as they go. That acid drops the brine's pH, usually below 4.6 within the first several days. Almost nothing responsible for food poisoning can multiply once it gets there.
This is why brine strength and keeping everything under the liquid matter more than anything else a recipe tells you. Both are just support for the same race: enough salt to hold back competitors while the lactic acid bacteria establish themselves, and vegetables pushed fully under the brine so the jar stays low-oxygen long enough for the acid to build. A jar that's under-salted, or has a piece of cabbage poking above the surface, hasn't necessarily failed. It's just given something other than lactic acid bacteria a head start.
Kahm yeast and mold are not the same failure
Kahm yeast is wild and uninvited, but it isn't dangerous the way mold is. It's an oxygen-loving yeast that colonizes the one place a ferment tends to have air — the exposed surface where a vegetable has floated above the brine line. It grows flat, thin, and white or cream-colored, more like a skin than a texture. Skim it off, push the vegetables back under the brine, and the batch underneath is fine. It can leave a slightly yeasty taste behind. It won't make you sick.
Mold looks different once you know to check. It grows upward instead of flat, in fuzzy or fibrous patches, and it shows up in colors kahm yeast never does — green, black, pink, orange. It also roots downward into whatever it's growing on, well past what a spoon reaches. Some molds produce mycotoxins that the ferment's acidity doesn't break down. A jar with true mold on it gets thrown out, brine included, not skimmed and kept.
Cloudy brine and a sour smell mean it's working
Brine that starts clear and turns cloudy within the first few days isn't spoiling. It's lactic acid bacteria multiplying fast enough to see. The same goes for the smell. A ferment turning increasingly sour, tangy, or faintly funky over one to two weeks is exactly on schedule — that smell is the acid itself, plus the byproducts of the bacteria producing it.
The one hazard a sniff test won't catch
Clostridium botulinum specifically favors the low-oxygen environment a packed, submerged jar creates, and it's worth naming on its own for one reason: it doesn't announce itself with a bad smell or an off appearance the way mold and spoilage bacteria do. That's exactly why the salt and submersion rules carry more weight than checking a jar by nose. A properly salted ferment that reaches a pH below 4.6 has already out-competed it. That race is the actual safeguard, not a smell check after the fact.
What actually means the jar goes in the trash
Slime on the vegetables themselves, rather than just sitting on the surface, means something other than lactic acid bacteria has taken hold. So does a smell that crosses from sour into genuinely putrid — closer to rotten eggs or spoiled meat than to vinegar. Mold that returns within a day or two of being scraped off means it's gotten past the surface, not just sat on top of it. Any of those three means the jar gets thrown out rather than tasted through, because none of them are things a spoon can sort out from what's still good underneath.