✦ Starter Troubleshooting ✦
Throw it out. All of it.
This is the one page here where the answer isn't reassuring, and it shouldn't be softened. If you see mold, the starter goes in the trash.
What mold looks like. Fuzzy or raised growth, in patches, often circular. It can be white, green, black, blue, or pink. The key word is fuzzy. Hooch is liquid, and a dry skin on top is flat. Mold has texture and stands up off the surface.
Don't scrape it off and carry on. What you can see on the surface isn't the whole organism, and mold can produce compounds that spread through the mixture you can't see. Salvaging a molded starter isn't worth the risk over something you can replace with flour and water in two weeks.
Wash the jar in hot soapy water before you use it again, or use a different one.
Why it happened. Usually contamination from a dirty utensil or the jar, food debris on the rim, or a starter left too long without feeding in a warm place. Keep the rim wiped, use clean spoons, and don't leave her unfed for weeks at room temperature.
Starting over. Flour, water, and about two weeks. It's genuinely not a disaster, and a fresh starter is safer than a rescued one.