Mould can sometimes appear on sourdough starter, especially if you haven't fed it for a while. If it has settled in your starter, you will unfortunately have to dispose of the entire starter. Mould propagates via spores and these can be present throughout the sourdough, even if it is only visible on the surface. As some moulds can produce strong toxins, it is very important that you really dispose of your starter and not try to save it.
You can clearly detect mould infestation: The mould on sourdough usually looks the same as we know it from other mouldy foods. It manifests itself as furry round spots that only grow on the surface and are usually white or dark green to black. Also compare whether it is perhaps only the harmless cream yeast. Unlike mould, it is not fluffy, but wrinkles and waves.