Mold and pink streaks.

Fuzzy growth on the surface, or streaks of pink, orange or red running through the jar. Almost everything else in this Atlas is fixable. This one is not.

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1. what it usually ISN’T

   Grey or dark liquid on top

   That is hooch — alcohol from a hungry culture. It can be grey, brown or almost black, and it can smell strongly of solvent. It is not mould, it is not dangerous, and the fix is to feed the starter. Liquid is not growth. If it pours, it is hooch.
2. what it usually ISN’T

   A dry, darkened crust around the rim

   Starter that has dried on the jar wall oxidises and goes grey-brown. It can look like a colony. Scrape a little off — if it is hard and flaky rather than fuzzy, it is just dried starter. Wipe the rim and carry on.
3. the real thing

   Mould — fuzzy, raised, three-dimensional

   Mould has texture. It stands up off the surface, it is fuzzy or velvety, and it usually appears as distinct circular colonies rather than an even layer. Colour is unreliable — it can be white, so "white so it is fine" is not a safe rule. Texture is the tell: flat and wet is fine, fuzzy and raised is not.
4. the real thing

   Pink, orange or red — bacterial contamination

   Streaks or patches of pink through the starter indicate bacterial contamination, most commonly Serratia marcescens. It is not mould and it does not look fuzzy, which is why people talk themselves out of it. Any pink, orange or red that is not from something you deliberately added means discard. This one has no ambiguity.
5. why it happened

   Too little acidity, usually from underfeeding

   A healthy starter defends itself by being acidic — the pH is low enough that most contaminants cannot establish. Contamination usually follows a long neglect, a very wet or warm environment, or a starter that was never fully established. It is rare precisely because a well-fed culture is a hostile place for anything else.

1. Confirm it is not hooch or dried crust

   Pour off any liquid and scrape the rim first. Then look at what is left. Fuzzy and raised, or pink/orange — discard. Flat, wet, and merely discoloured — feed it and see how it looks tomorrow.
2. Discard all of it, not the top layer

   Mould sends hyphae well below the visible surface, so scraping off the top does not remove it. There is no safe amount to keep. Bin the contents, and do not compost it indoors.
3. Wash the jar properly, or replace it

   Hot water and detergent, and a dishwasher cycle if you have one. Scratched plastic containers hold onto contamination in a way glass does not — if it was plastic and scratched, throw the container out too.
4. Start again, and change what caused it

   Building a new starter takes about two weeks. Before you do, work out what let it happen: an unwashed jar, months of neglect, a warm humid spot, or a culture that never got fully established. Feeding on a schedule is the actual prevention.
5. Next time, keep a backup

   This is the argument for drying some of your starter. Ten minutes of work and a jar of flakes in a drawer turns this from starting over into a four-day inconvenience.